How many and how heavy of container plants can a rooftop deck handle?
While I realize that it's disappointing not to have every question easily answered by the internet for free, there is no way to correctly and reliably answer this question than to consult with (yes, pay money $$$ to) a civil/structural engineer, preferably one licensed to practice in your local jurisdiction (State, since you are in the USA.)

For 5 different houses, built differently, there will be 5 different answers. There is no "General" answer, and the stakes are at least your house, if not the lives of your family if you get it wrong and collapse the place while you are all asleep, etc. It is NOT a good place to "guess."

Joining a community garden near your house might make more sense as a way to get garden space.