Modeling database

Hello,

In my site users will choose one subscription to have a certain amount of
properties to advertise.

For example, if he bought a subscription with 5 ads, he can advertise 3
houses and 2 apartments.

The user can also modify the subscription, for like 10 ads. Now the same
user can advertise 5 more properties. But he can also decrease the amount
of ads, for example 2. But in this case he will have already more than 2
ads running.

Is there a way for everytime the subscription changes, if he has more ads
than the subscription allows it just turn off the amount that is more than
hired?

I'm not sure how to model this problem yet. Any answer would be awesome.

Thank you