Never have played TCP/IP myself so can't answer that one. Most likely the limit would be 20 though. It is everywhere else in the game.
Most people play using PBW.
http://www.pbw.cc
It works great. I have played many games using it. It can take awhile to play through a whole game but it is much easier to get schedules etc to fit so that it works for everyone.
Hrmm... How do mulitplayer games work. Not sure quite what you mean... but for PBW it would work something like this.
Each player downloads/is e-mailed/ one of a few other options their turn file. Each person plays the game on their machine ends their turn and the game creates a .plr file. You send that via whatever method you are using to the host server. Once all players get their turns in or the time limit for that game is reached or whatever other setting for kicking a turn off is reached the PBW server processes the turns and sends out the next turn files to everyone and updates the page and allows for downloading the turn file from their as well. Rinse and Repeat.
As for the actual game. PBW only supports simultanious turn games so each player basicaly issues the orders during their turn.
Tell ship A to move here
Tell ship B to attack ship X
etc
instead of seeing the ships move they accept the orders and when the turn gets executed everyones ships execute their orders in a stepped format of 30 cycles (every turn is 1/10 of a game year or 1 game month, 30 game days in a game month). Ships with more movement points will move first and more often. IE: If you had a ship that had 30 movement it would move once every game day however if you had a ship with 15 movement it would move once every other gameday.
If you so desired during your turn you could playback the movement log and see the ships move if you wanted. You get combat logs and can't directly fight combat. Instead you use the strategies and set them up. This feature is very important for PBW.
Hope that answers some of your questions and gives you a general overview.