There's no specific limit of how many generals there can be, and any rank of the WR for that matter. The exception is that there can only be three Three Leaders of the World as name gives away, but that's the only rank that has a limit, and people can't be promoted to it or demoted from it.
The amount of Generals is small though, smaller than any other rank, and to achieve that rank you have to have done something really exceptional.
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Nio-chan and Ctes: http://img.imgcake.com/nio/Untitledpngyn.png
BroAndCharAndCtes: http://i.imgur.com/oBQfY.png