The Buffer Publish plan that you're subscribed to will determine how many posts you're able to schedule in advance.
|Premium, Business, and Enterprise plans||2,000|
This is not a daily, weekly, or monthly limit, this is the number of posts you can store in your Queue at any one time. For example, if you're subscribed to the Pro plan and you already have 100 posts in your Queue, you wouldn't be able to add anymore until your total Queue count is less than 100. Put another way, if you share 10 posts per day, you'd be able to schedule up to 10 days in advance on the Pro plan.
It's good to keep in mind that we also have daily posting limits in place for each social network, which you can learn about here.