As of October 10, 2017, scheduled messages will be limited to 30 at a time for Free users.
How does the limit work?
Continue scheduling messages as usual up to the limit of 30. As soon as one of those scheduled messages is published (or deleted), you’ll be able to schedule another one. The compose message box, Composer, Hootlet, and Publisher calendar will now display counters to help you keep track of what you have scheduled.
Note that messages are counted per social profile. For example, if you schedule a message to three of your profiles, that will count as three scheduled messages.
See this article for details on editing or deleting scheduled messages.
If you have more than 30 scheduled messages on October 10, when the limit goes into effect, your messages will all publish as expected. However, you won’t be able to schedule additional messages until your counter gets below 30.
If you need to schedule more than 30 messages at once, upgrade your Hootsuite account to unlock more.