When sending mail, there is a limit to how many recipients you can send an email to at one time. This limit is 4,096 bytes, or four kilobytes. Please note that this is not a limit on the size of the actual email itself; this is a limit on the size of the portion of the email which holds the recipient's email addresses (e.g. the "To", "CC" and "BCC" fields).
Here's how it works: each character in an email address, excluding commas, semicolons and spaces (which are used to separate multiple email addresses in the same field, depending on the email client you are using), uses up one byte. Therefore, if your email addresses are twenty-two characters long on average, then one email can be sent to as many as 186 recipients.
In the event that the email addresses exceed the four kilobyte limit, that portion of the email is truncated (shortened), so that it will no longer be over the limit. This means that some of the people on the recipient list will not get the email you sent.
As a general rule-of-thumb, you will be more than reasonably safe if you never send an individual email to more than 100 recipients at any one time.