I'm sorry, it appears English is not your prime language, and it's OK, but I do not understand what mechanic you are looking for exactly.
You talk about a limit of 5 bullets. So you should look into global and/or instance variables to count the number of bullets (or use the dedicated expression "Count") you have (on screen for example ?) and when the value is 5 do something.
I don't know what you mean by "activate bullet". Enable/Disable them ? If so, use the appropriate action from the Bullet Behavior.
In fact, it surely would be easier if you could provide your current project to let us know what you have so far and what is the context of coins and bullets you are talking about.