@pdegro welcome to Photrio!
No, you don't need 20 posts to put an ad in
the Classifieds section - you do need a (paid) subscriber account, though. Please see here on how to obtain one:
https://www.photrio.com/forum/pages/supportus/
Also, please refrain from making posts just to hit that 'magic number' of 20. It's really not necessary. In general we're quite willing to help you out if you want to do something that requires this post limit. It depends a bit on the situation, but it's easy enough to approach any of us moderators and explain what you're running into.