Are there limits to drug resistance?
There are physical limits to the existance of life forms, wether temperature, pressure, osmolarity, etc. But these are usually physicochemical fields acting over a wide spatial structure. In the case of drugs, they are physically localized molecular entities. They are usually very tiny, even compared to the smallest microbes. Even though the genome of any microbe is finite and they could not, in theory, recognize the infinitely many drugs we can synthesize (assuming the simple case they need one gene per drug resisted), they can just evolve very generic mechanisms like preventing these molecules to get into the microbe, pumping them out of the cell, or other generic mechanism; with the same overall effect: preventing the drug from acting in the desired way. 

The question you ask is very interesting, and it is the focus of some recent efforts and discussions among academics. Here you can find a recent article discussing the possibility of "evolution-proof" drugs. As you can see, the subject is not trivial. 

Cheers,

Pedro