A Commander deck consists of 100 unique cards, of which one is designated as the Commander and must be either a Legendary Creature or an allowed Planeswalker. I'm still very unfamiliar with this format. How would you go about figuring out the number of lands to put into your commander deck?
Because of the slower pace of play in EDH/Commander, it isn't as drastically important to hit 5-6 land drops in 5-6 turns. In addition, the number of "mana rocks" (mana-generating artifacts) and dual lands/mana filtering/land fetch cards are usually higher.
Considering that, the 40% lands rule is still fairly close to normal (lightened obviously for the reasons above). I've run 34-36 land decks successfully, but usually you see 37-40 with some of the previously mentioned accelerators.
Abnormal decks (with huge numbers of accelerators) will run as few as 32 lands (the equivalent of running about 13 in a 40-card limited deck), but I'd recommend starting around 40 and cutting around one land per three accelerators (if you're trying to cut land, and not running very high-cost spells).
Do also remember that running fewer lands with more mana rocks leaves you open to blowouts when someone wipes artifacts with Austere Command, Merciless Eviction, Shatterstorm/Creeping Corrosion, etc. - you don't want to be stuck on four lands when your couple of Signets and Thran Dynamo get wiped.
As an example, the Devour for Power Commander deck shipped with 40 lands and four mana-generating artifacts, but it is fairly heavy on costs.
This is actually a more complicated question than it sounds. There's a lot of things that go into deciding how much land goes into a deck, the general rule for minimum is 1/3 of the deck, so in EDH that would be 32 cards, but that is a pretty bare minimum and most decks will need more than that. Lets go over the things that determine how much you'll need one at a time.
The more colors you have in a deck, the more land that deck will need - a mono color deck doesn't need specific lands to play their spells, my Marrow-Gnawer () can get away with a lot less land than my Nekusar, the Mindrazer (), which can get away with less than my General Tazri (). This is mostly because you need to get your colors on the field sooner, and the more lands that can make each color, the more likely you are to do this. It's always a bad feeling to never hit that last color and have a hand full of things you can't cast. Lands that can tap for multiple, or any color blunt this, but have downsides, and are always one Ruination or Blood Moon away from keeping you out of the game for good.
With a mono color deck, you will be getting all of your colors usually on your first turn, with 3 colors usually takes 5 or so mana generating cards and in all 5 colors you can still be waiting for that last color sometimes after a dozen lands are already out.
There are some exceptions to this, for instance I have a Reaper King () deck that most cards are artifacts and don't care about the colors on the field, letting me trim some land from what would normally need to be more land heavy to support all 5 colors.
What's the average cost of your spells? How many X cost spells and abilities do you have? If your deck doesn't need much mana to run, missing land drops won't be as big of a problem, if you're playing something like Dragons which tend to be expensive, you'll need to get your mana base set up pretty quickly and can't afford to miss as many land drops too early on.
Yes EDH is a slower format, but having to come from behind and catchup to your opponents does put you at a disadvantage when they play spells and you can't.
Non-land mana sources
Land isn't the only source of mana you can have in your deck - Elves can get by on less actual mana than some other deck types because so many elves themselves can tap for mana, both accelerating your deck and making more land coming less desirable or useful. Creatures are, however, easy to destroy, particularly as many of the mana producing ones have low toughness like Birds of Paradise or Llanowar Elves.
Artifacts that produce mana are common too, there's no single card in more EDH decks than Sol Ring, some of the moxes and medallions can be pretty helpful too. These like creatures are easier to destroy than lands, and aren't a replacement for them. The medallions lose some of that usefulness pretty quickly as more colors are added, it's not usually worth 5 slots in a deck, but fit very nicely into a deck with only one or two colors.
Other Ways to Play
Paying the cost of a card isn't the only way to play it, and if your deck uses one of these strategies you won't need as many land on the field at once. Jodah, Archmage Eternal () makes every spell you cast able to be played for , Kaalia of the Vast () lets you cheat in Angels, Demons and Dragons whenever she attacks, and there's always cards like Elvish Piper and Quicksilver Amulet. Reanimator is another option, getting cards in your graveyard so you can Reanimate them later using cheaper spells and abilities reduces the cost to actually play a deck, and thus the land base you need to build up.
How are you planning to play your game? Aggro isn't the strongest in EDH but there are ways to make it work, and that needs to have a lot of mana, fast. Control too needs to get it's mana base stable and early so it can afford to hold those counter and removal spells to use when things start happening. Other decks can afford to build up a bit slower, unless it's forced EDH is a slow format after all, the faster you want to act in the game, the more land you likely need to do it.
There's no one solution, generally you'll want somewhere between a third and half of your deck to make mana, and most of that will likely be from land. Even if you are running Elves with mana rocks, you need the land first to play the elves, to play the Sol Ring, before the non land mana engine takes over, and you can't rely on drawing enough land to start your game if you're running only 20 in the deck.
It depends on your deck, and the pace you want to play. Are you trying to turn 3-5 combo out the table, are you trying to use politics and hug the table only to backstab your allies an route to victory, or are you a power gamer trying to drop the fattest fatties and smash your way to victory. Plenty of strategies in edh and as such it is color identity of your commander dependent, cmc of your cards dependent, and desired pace dependent. In a trying to combo out asap deck I run limited lands around 24 lands with mana rocks galore and then tutor and my 2 or 3 ways to combo out. The rest of the deck is removal so if my strategy slips to turn 4 or 5 I'm able to defend myself from table politics. If I'm just casually playing I run about 40ish lands and some rocks as land destruction is a thing aforementioned in the thread. So for decks in between these extremes you have to just kind of play it by ear. Build it play test and tweak based off hour results. Also try an play test against as many play groups as possible. I go to 5 different shops for drafts, deals, and of course edh and you'd be amazed how many tables are just a few people playing edh for fun that will welcome you to come test your deck against them. Also if you're gonna go for the atraxa, kaalia, and other obvious hate generals don't forget to build up protections in your deck and don't get mad if you get aimed.