Roadie?
I’ve always pinned the numbers over my pockets. Good enough for the pros and you can add more for aero gains and stopping them pinging off when rooting in your pocket for a gel.
TT skinsuit or speedsuit – nopinz or get the tabs sewn on so you aren’t pinning to the skinsuit itself.
CX and Hill climb arm numbers – elastic tied to the holes can work but again no pins make something or you could fasion an arm band in the same way.
The magnet and plastic clip things seem to come undone from what I’ve seen.