You really only need one per pump if you're running 24 volts. You could always mount up two per pump and only use one. That way if one ever went out, you could just swap the wires over the the other for for a quick fix. However, I haven't had one go bad in several years running them this way. Just make sure they are grounded well... no self tappers through the sheet metal!
I have them available and in stock, just copy and paste this link...
http://www.dicesuspensioncomponents.com/store#ecwid:category=2507838&mode=product&product=11979811