Hosting problem with more than 9 players?

[thevoden]

I have a fast broadband dsl connection. I can upload 30 kb a second max. Well thats pretty slow but its still broadband!

I can host games normally with 6 people. But whenever I host games of 9, 10 or 11 it seems that people are experiencing lag spikes and such. I forwarded my ports correctly and such.

Any tips?