As Pat Ludwig states, position is important in the game. So you always want each table to play a full round so everyone plays from all the good and poor positions. Pat Ludwig's suggestions are good advice under the assumption that you want a quick tournament and that all players arrive on time for the start.
Fitting all the matches into an appropriate amount of time is a key success factor for a tournament director. Plan your rounds starting from final table backwards. I will assume this game takes ~10 minutes / player / round once everone knows the rules. Under the assumption that you have a little more time, I perfer a drop the low men approach.
I also like having your final table start with a full seven players. Drop from 1 to 4 players after each round depending on time available.
For early to middle rounds in a really large tourneyment, I would suggest around four or five players to a table. More players makes the rounds slower, fewer players makes the outcome too dependent who you face in the round.
Each assignment of players should play exactly once around then break for new assignments. Drop X players after the round then reassign seeding by chip count.
So my example 20 player 4+ hour tournament would be:
First round 5 tables of 4 (~ 45 minutes)
Second round 4 tables of 4 (~ 45 minutes)
Third round 4 tables of 3 (~ 30 minutes)
Fourth round 1 table of 7 (~75 minutes)
Fifth round 1 table of 5 (~50 minutes)
Sixth round 1 table of 3 (continue as time allows, place per chip count at end of scheduled time)
If you want to allow participation by late arrivers, here are some ideas:
1. Reserve a table for late arrivers. But don't wait too long to get this table started.
2. Have players pair up for an initial seeding round as they arrive. Only play two hands in this fashion use chip count to seed players for the initial full tables of the tourneyment. This allows some time for late arrivers and helps insure that folks know the rules, before the beginners slow down a larger group.
3. In addition, Charge some minor fine on initial buy in for late arrivals. Probably a little less than 1% per minute late.
Initial seeding is also a significant fairness factor. Each starting table should have an even mix of beginners and experienced plays to the greatest extent possible.