There really should be no more than 5 people, because you don't won't 15 or 20 people tagging along. You will get slowed down and might cause too much noise and attract zombies. You're also never going to know how many people have been bit. And chances are everyone is not going to have a weapon so those people are just going to be supper for the zombies most likely.
In my opinion you shouldn't travel alone because you never know what's going to pop out from behind you. You should always have someone watching your back. It's way more efficient on food, ammo, and supplies if you have less people.
If you must have lots of people following, split up into smaller groups of 5 or 6 and have a route set for each group to your destination. Use walkie talkies or some sort of 2 way radio so you know what’s happening all of the time.