as long as I know, until now there is no limit, you can post as much as possible, but of course must be approved by admin
@hafiz786 is absolutely right. You can see this limit on ‘Earn money at OC’ page.
We can ask as many questions as much we want. But, according to rules and regulations we can only get points for our first 10 new questions per day. There is 10 points for each new question. So, we can earn up to 100 points only by creating new questions on daily basis.