For years I did plenty of flights moving a few altimeters back and forth on launch day - I would prep all my ejection charges and sleds for the rockets I wanted to fly and after each launch I would move the altimeter, connect the prepped charge leads, check battery voltage (replace as needed), and fly again.
I have a fairly common av sled design and switch wiring so I set each rocket up to fly on 2 or even 3 different altimeters without having to change the sled or switch. Extra altimeter mounting screws are far cheaper than a fleet of altimeters.
I'd rather trade altimeters between airframes and buy more motors instead of buying an altimeter for each airframe. I have an assortments of altimeters now, acquired over time based on specific flight parameters.
Min diameter? Raven 4
Large staged flight? Marsa33 with tilt gadget
Going over 20,000? XTRA
Basic DD? RRC2+
Dip your toe into DD and buy something for what you need and what you want to try over the next year. Once you find your groove and get a brand preference, your inventory (and associated capability) will grow naturally over time.