I'd say one or two for every computers you own is a good number. If you happen to have lets say 6 boards with 2 computers, consider trading instead of buying; you will eventually try many different things without paying them all and you have sufficient to always type on something worth.
Right now i only have a single board and 4 computers (Job computer, school laptop, gaming rig and chromebox) which means i'm in dire need of getting new stuff lmao.