[Publish] What does "number of scheduled posts" mean?

The plan you're subscribed to will determine the number of posts you're able to schedule in advance.

This is not a weekly or monthly limit, this is the number of posts you can store in your Queue at any one time. Let’s say you are on the Pro Plan and already have 100 posts in your Queue. You will not have the option to add any more posts. However, as soon as one of your posts gets published, you’ll be able to jump in and top your Queue back up to 100. If you share 10 posts per day, you would be able to schedule 10 days in advance on the Pro Plan.