not sure how anyone here can tell you a safe amount..once you need extra strong glasses to see to user accounts on start up because there are so many rows, is maybe a good time to stop, then you get new PC/laptop for the rest or another OS on vm ware....
Agree with turtleboss - if you do stealth properly - and i mean to a T - there is no reason why you can't have many accounts on one computer. Only issue is having to change each time to access an account and keeping all your details in order.
What I usually do is keep all my active accounts on 10-20 users then I'll keep all my backups on 1 user which can be unlimited and just clear flash cookies, history each time I log in then keep all the account details on win notepad saved on my bbery so then I have acces to all my account info from any user.