You need 30 points to be able to post on a wall, even your own. This was implemented a while ago to stop people's walls from being spammed by robots. Since you start out with 20, the extra 10 should be very easy to get. Just one vote will get you there.
Note that if you have 30 points in PokeBase but not Meta or RMT, you will only be able to post on walls via PokeBase and vice versa with other sections. This may or may not be a bug with the site.
Also, you can add an infinite amount of posts to your wall. Putting too much on your wall should not affect your ability to post on walls. The post you read was most likely describing how the site blocks you from adding wall posts if you post too many at once (also implemented to slow down spambots).