If we have reached our 100 submissions for the month will we be notified or do we just have to monitor it ourselves?

You will be notified by email as you approach any limits of your plan, such as submission count or uploaded space.

You will also be notified when / if you go over the limit, when further form submissions would be disabled until the first day of next month when the counters reset.

I hope this helps. Please let us know if you need any further help or clarifications, we will be happy to assist.