It is not really possible/usefull to give you any statistics on this for now. This is due to there being so many factors that all play a role. For example, consider this:

• What is your definition of smoothly (I would most likely not even consider 1 database to run smoothly on that system) ?
• How many users are present in a single database that login?
• What is the average task of a user (I can imagine creating a sale order as not really processor-consuming, but creating complex reports might slow down a lot)?
• How many records are the tables on average?

Etc. etc. etc. I think it should be trial and error in this case.