As I understand it, the second isn’t an issue so 50 vaccines over 50 years would be fine, that’s how seasonal flu vaccines are done.
The first is mostly a question of how similar they are. Best case original vaccine covers all strains without modification, worst case you need 50 different vaccines. But the most likely combination is a mix where you might need say 5 vaccines, each of which over an overlapping subset of the 50 strains to various degrees of effectiveness.