Volume and pressure are the two main factors. Too small of a pipe and you can maintain pressure, but not supply volume. Too large and you can achieve volume, but perhaps cannot maintain pressure.
IMO, for 50 drops, 3/4" or 1" depending on distribution of the T's. Closer together or shorter run, go with a larger pipe IMO.
Distribution of the T's matters. I have 3/4" pipe (originally sized it for a smaller pump) and run nearly all of my T's wide open (max airflow). If I run 10 T's, each 6" apart, I have no issues. If I were to run 10 2" apart, I cannot maintain the same output pressure across all T's without fiddling with them.