It really depends on how many "users" will be accessing the internet at the same time. If you have 5 computers set up and only one is using it, the speed is not affected. However, if all five are downloading files then your the speed would be devided between them.

At my work, we have 25 computers hooked up to a single DSL line. During a normal day, I rarely see any difference in speed because the usual activity by everyone (not all at once) is browsing web pages. Not downloading large files.