Depends on the mower. i.e. how many it needs for however many dimensions it moves in - typically 2, but maybe 3.
I put a "pack" of 4 on my smallest tractor for about $500 a decade or so ago.
Surplus supply had Parker units in several configs, I wanted 3 at the time, but 4 wasn't much more $s
There was some reading on their web site at the time, enough the get me "educated" well enough the chose what I needed.
BTW, a handy place to mount a bank of 4 remotes is on the right side upright of the rollover bar.
It works well for things like stump grinding if you want to be OFF the tractor to not get vibrated and it is next to the loader joy stick so I can inch the tractor back/forth with the bucket curl//dump.