This what it says if you read the "Limitless Pictures" in the "Subscribe" section
"We only limit people to 10 waiting jobs at a time (to stop anyone getting a monopoly over the system). Beyond that, you can take as many pictures as you want of whatever you want."
So, once you have submitted your 10 jobs you need to wait for one to be completed before you can submit another. This can be a bit of a bind if you are imaging some unpopular out of the way object but BRT will get around to it at some point. On the other hand you could be lucky and have your jobs completed pretty quickly, as I did with 6 of mine last week. In which case you can immediately resubmit further jobs up to the maximum of 10 an wait for more to be completed.
You can read about the changes to the scheduler that have recently been made and I read somewhere that the average wait is about 6 days so you may well get more than 10 images a month.