There are two main limits on sizing cable. Thermal is the same regardless of length. It basically comes down to how much heat is produced per meter of cable, vs how much heat can be dissipated per meter of cable. This doesn't change whether it's one meter or 100, although it is affected by environment (clipped surface, in a wall, surrounded by insulation, underground etc.). On short lengths, this is the main determining factor.

Second is volt drop, generally you want no more than about 2.5% in a subcircuit. The longer the cable, the more volt drop, and eventually you'll have to go up a size.

The reason you can have stupid numbers of hotpoints on a circuit is that if the user tries to load them past the 16/20A of the breaker, the breaker cuts the power and lets the cable cool down, given some caveats around sustained loads, trip curves, and accuracy.