I'm guessing around 15-20ish, although you may start getting jumpy graphics before you encounter problems with your connection...

well i got a linux root server hosted in a data center in germany

1 GHz Pentium Celeron
256 MB RAM
Linux

100 mbit Ethernet

75 gb traffic free / month

do you have any idea how much traffic i should except from one client per month so that i could calculate how much people should be allowed to connect to the server? also i wanted to know how much zones would be able to run at the same time (because of the limiting cpu power)

I believe it is around 5Kb * Number of Clients

So 5 clients is 25Kb upload for no lag per second.

That's 1.5Mb per minute.

That's 90Mb per hour.

That's 2.16Gb per day.

That's 64.8Gb per month, for 5 clients connected 24/7. Most people wont be playing 24/7, so I'm guessing it would be about 65Gb for 20-25 people to play on your server regularly. It sounds like your host can support that...