Correct - the amount of consists will determine how many players can join the session.
One of the buttons on the toolbar at the top of the Driver interface (when in a Driver session) will allow you to host a session on said route/session, where you can specify how many players can play the session at the same time.
Shane
		
		
	 
Not entirely true Shane. The number of consists limits the number of people that can drive. Haivng 6 consists means 6 players can drive, but you can set it in the rules to have up to 10 people join. 
When setting up a session, think of how many trains will need to be ran. For my MRL route, I know how many jobs there are. There are 4 yard jobs, 3 manifests, 2 passenger trains, 5 locals, and a branch line. So I need at least 14 people in my session to run it right, meaning I should have at least 14 locomotives and drivers ready for players. I tend to put more in just to be on the safe side. As for cars, I tend to put 4 cars per industry that it serves per yard and one at said industry. So if I have 1 grain silo, but 2 yards, Id put in 12 covered hoppers. 4 at the grain silo and 4 in each yard. But then again with MRL, I put many more in, just in case. Also, placement of cars is important. While I do tend to put cars in some odd places in the yard's, that's more for staging and operational interest. I tend to try and put cars where I know they will be needed first. So on MRL in Billings where reefers are loaded I place most of them. Same with auto racks and box cars. I know my Billings crew will need them right away to get working. So I left them a train to work with. 
But then again, its all on how YOU want YOUR session run. I want freight trains moving goods back and fourth making my railroad money. So I place the cars in the yard, then place the locomotives else where. Letting my "crews" make up their own trains how they want or based on where they are going. But maybe you want to have passenger trains scooting around moving people from town to town. So you build the trian locomotive and cars all ready for your "crews" to take off with them.
But once more, YOUR session, YOU set it up how YOU want it ran.