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What kind of connection? What's the nominal bandwidth, both up/down if asymmetrical?
You're correct, TCP has many factors impacting its performance. You can't really assume what bandwidth throughput you might obtain unless all parameters are known and controlled. You might try one of the speedtest Internet sites.
Are there any firewalls or proxies in-line? They usually only cause a minimal performance impact, but that's not always the case.
As to whether 6 Mbps is the correct or sufficient bandwidth, a simple rule is if network is waiting on you, you have the right amount of bandwidth. If you are waiting on the network, you could use more. Simple rule, and the latter is often the case but unfortunately, the cost of the latter often has us as the former.
From a business perspective, if the Internet usage has a business purpose, measure the cost of paying employees to wait vs. the cost of the additional bandwidth. Sometimes your overall costs can be reduced by paying for more bandwidth and making employees more productive.
Another simple guide, what are people used to? If your users have 10x better performance on a high speed home connection, poorer network performance on the job can negatively impact morale.
Everyone, of course, generally prefers the fastest system/network performance, but again, in a business perspective, you need to balance cost against productivity. Only then can you "right" size. In other words, there's no one right size of bandwidth for x number of users.
