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Control points may belong to more than one control point round.
You could theoretically, have up to 8 rounds with a CP in each round.
Now for one of my wacky ideas: you could try and recycle the use of control points, so that you could have, say 6 rounds with 2 CPs in each of them by reusing and redesignating ownership of previous control points. For example CP_1_A and CP_1_B could also be in round 5/6 simply because, according to the Valve wiki:
Therefore, with correct usage, you could create a map with unlimited rounds (or, up to the limit of team_control_point_rounds)
I would like to see a custom TC map, haven't heard of one being made.
But I took a look at the hydro vmf and can understand why, it's a lot of work, something like 10 rounds, for every round the are fences that need to be enabled, doors that need to be opened/closed, signs that need to be changed.
Maybe start with a simple TC, like just 4 areas, which would mean 6 rounds, even then that would be a fair bit of work.