Its an interesting calculation that can assume some relevance if related to a specific case study (find next sexy prime number). A prerequisite would be to konw the processing power needed to tackle the problem in a definite amount of time. Once you have those contour parameter set you can estimate the scale of the botnet.

and last: botnet >> supercomputer in term of scalability since its cost-free and becouse a supercomputer can always be part of the botnet, not the other way around