This is less a question of business letter etiquette and more a question of e-mail etiquette.
The answer is: technically, no there is no limit, or if there is one it's so high you're unlikely to ever reach the limit. However, it's not good etiquette to put too many addresses in the To or Cc lines, especially if the recipients don't know each other.
For example, if you send an e-mail and send it both to rewboss at rewboss dot com and to dishonest-businessman at shady-dealings dot com, then dishonest-businessman now has my address, although I might have wanted to keep it secret from him. Similarly, if you were, say, asking for an estimate for some job and cc'd the request to rewboss at rewboss dot com and jsmith at rival-company dot com, then we (jsmith and I) both know that we both got the same e-mail, and that can cause problems.
A separate danger is illustrated by an anecdote related by an online acquaintance of mine. Someone at the company he worked for received one of those chain e-mails and decided to forward it to his co-workers -- not just to the ones he knew, but to every single employee in the company (numbering in the hundreds). That was bad enough, but then a large number of these people replied to him to tell him how stupid he was, but they did a group reply, which meant that within minutes, several tens of thousands of e-mails were on their way; and of course, some people decided to group reply to tell everybody else to stop sending stupid e-mails, and eventually the mail server crashed.
Only cc if you have to, and only cc the people who really need to see your e-mail.