That depends on your e-mail server (BellSouth, Yahoo, MSN, etc.).
I would break it down to no more than 250 at a time, but that still may be too many.
I believe the limit is 50 addresses per message. However, Yahoo also has a limit of only 100
E-mails a day. They have gotten very strict about spam, and people sending spam. They will suspend accounts for what they believe are violations of “suspicious activity” on your account. So take care how you set that up.