Are those 6 Mbps guaranteed or it is an explanation in which they say "up to 6 Mbps"?
If you can afford more than 6 Mbps it would be better. Are you sure that sometime there will be 18 users connected to the internet?
I agree with Deerpark regarding the quality of the router - pay attention, the router has to deal with many connexions, it has to be a very good product (NOT a "for home" router, if more than 10 people will connect at the same time).