Making a counter from 50MHz Clock?

The "others => '0' is equivalent to writing:

count := "000000000000000000000000000"; -- and I lost how many zeros.

The subtraction is not to reset count to zero and you shouldn't. Instead you wrap up and keep difference from zero (count phase).

To understand it better take case of generating 21MHz from 50MHz clock.

the ratio factorises to 21/50 so:

0, + 21, + 21, + 21, + 21, + 21,... =

0, 21, 42, 63, 84, 105... and with modulo 50 =

0, 21, 42, 13, 34, 5...

0, 0, 1, 0, 1, 0... MSB will togle at 0.5 or more of count value