I'm looking for a couple different things here.
From what I can find, Workgroup edition supports up to 25 subscribers per publication. Does anyone know the limit, if there is one, to the number of subs per pub in both Enterprise and Standard editions of SQL 2008 R2? (2008 info is good too)
Also, anyone have any experience with 50+ subs against one pub? We are looking to get some ideas on what performance looked like with that many subs. Did you need to add an additional pub to handle the load? Were there issues when a certain number of pubs tried to sync at the same time?
Workgroup edition is the only one with a subscription limit (25). Subscription to snapshot publications, however, are still unlimited on Workstation. Standard and Enterprise do not have these limitations at all. This has been the case since at least 2005, that I know of. (Source)
The performance of subscriptions are based on the number of transactions, obviously. As with everything "capacity" in SQL Server, it's all a matter of testing performance. There are so many variables to take into account that any formulaic answer would just be a wild guess. 50 subscriptions is not unheard of at all, though, so don't worry that you're going to break SQL with it. Anecdote: Another team here has a few pubs with 75 subs and they're only running a single server with no real issues.
Edit: a SQL guru coworker just pointed me to this Technet link on performance with merge subscriptions. It is written for 2005, but mostly applies to 2008(R2).