Each account can contact up to 200 individual people in a 24 hour period. This is based on the number of people contacted, not the number of followups sent. You can contact those 200 contacts as many times in a day as you'd like (send 2 texts, 2 voicemails, and 3 emails, for example), but sending a single message to 201 contacts in a 24 hour period will cause that last message to fail. This limit helps to ensure compliance and prevent your account from being flagged as SPAM.
The 200 contacts per day limit applies to each account and is a hard limit. This is an uncommon limit to encounter, but something to keep in mind if you're uploading a high volume of contacts.