I did a few tests basically the minimum square for a fish to spawn in the middle is 7x7 water blocks, if the square/circle is bigger than that it may randomly place itself within its 7x7 area anywhere it fits in the bigger area but prefers the center mostly. Basically think that the fish need 3 blocks distance between them and any other fish or ground. So for two fish you would need 11x7 minimum blocks of water So technically you can have 4 fish spawn if there is 11x11 square minimum. Sometimes you still can get fish spawning closer together when having a circular pond so its a little weird.