Each ad stays up for 14 days, so if you post all the ads that you are allowed on day 1, then the ads will autodelete after 14 days and on day 15 you should be able to post more.
If, however, you post one ad a day then you will only be able to post one more ad on day 15, then another on day 16 etc.
The classifieds works on a total number of ads allowed per forum user and when they have run their course you can post more.
This is to prevent a small minority of users abusing the classifieds by posting repeated duplicate ads as they have shown that they are prepared to do when the limit is not present.
Insecurities are about as useful as putting the pin back in the grenade.