I do not know, having not played this game for a long time. IIRC, I was capable of hosting up to 10 players without any serious lag on my end, if I had nothing else open.
It is, truly, impossible to know, since I do not know how much control-lag other players had. Other factors are also to be included; net-usage, other people connected to my router, connection speed of clients, usage of clients internet connections, distances from me, line quality, etc. At the worst, 5 players. Best, 10.