18 lands, including all the on-colour heal lands you can get and all the tri-lands in at least 2 of your wedge's colours. Then add banners according to deck speed and colour balance.
The thing to consider with trading Banners for lands, as with any colour fixing rock is "what happens if you don't hit the mana on time". Between morph and delve, Khans is a format where less than 1/3 of the common/uncommon cards in the set require more than 3 mana to cast.
So if you miss your third land drop, you are going to be immediately behind and your opponent can start capitalising on that. And if when you finally draw your 3rd land, your first play is a banner, that's one more turn that you haven't done anything to slow your opponent's pressure or increase yours. If that banner had been a land instead, it would have got you to three sooner.
Beyond that, there's another threshold at 5, where creatures can unmorph that can actually just kill other morph creatures rather than trade. Again, once your opponent gets there first, you can't profitably send your morph creatures in any more.
So the answer is "if I stick on 2, I can't play the banner, and I'm probably going to lose". So you want to avoid that. Your risk appetite may vary, but here are the stats.
Sealed is of course one of the more bomb focused formats, but one of the wonderful things about bombs is that they can wait a few turns till you find your splash, as long as your opponent can't kill you in the meantime. Which is exactly what will happen if they're hitting you for multiple turns because they were on curve and you weren't. They'll also hit their bomb faster.
Note also that a lot of the Khans cards are mono-colour, and the common multicolours all have morph. You're rarely going to require 3 colours until turn 5 or 6, and even then, you could well have stuff to do if you've built your pool to focus on 2 colours with a 3rd splashed.
And finally, remember that to cast a multicolour card you still need the mana. Having 4 mana in three colours does not help you cast a Zurgo, because he costs 5.
My advice, from listening to the Limited Resources podcast, is 18 lands and 0-2 banners depending on your fixing requirements and your deck speed. The more you can focus on 2 colours, the less you need the banners. The more controlling your deck is, the more it can afford the tempo loss of playing the banner out. But you're playing the banners as a patch to handle a lack of colour fixing with an option to cycle, not as lands. You might be able to go 17/2, using the ramp and cycling from the banners to mitigate any missed drops, but 17/1 is going to hurt more than it helps.
If you can play a mostly black-white warriors splashing red for Zurgo, out go those banners. If it's blue-black-green delve that Scouts for key tri-colour cards and needs to hold the ground then pour in the mana into Villainous Wealth, in they go. Remember, if you're mana flooded, you always can crack the banner on your opponent's end step.
Obviously, the better your land fixing is, the less you need the banners. Also remember that an off-colour tri-land that covers 2 of your colours (e.g. Abzan triland in a Mardu deck), counts as a heal land for the purposes of fixing.
Addendum: Based on post-pre-release discussion and comment, my initial opinion would seem to be borne out. The LRR team suggest that "You have much better things to do on turn 3 than cast a banner", and you'd normally do it only to help in a situation where you need more than fixing than your dual/tri-lands can be provide. This is always a bit of a judgement call, even if you can work out the probabilities, the consequences will depend on your deck and your opponents (e.g. you'll get more punished for colour screw by an aggressive opponent, but your deck may be able to stabilise better without the banner).