Depends on the wheel. A freewheel hub will accept a slide on (to the threads) protector with no spoke count involved. A cassette hub almost always will have a protector that engages the spokes (with the plastic tabs secured on spokes) and thus is spoke count dependent. Since I don't remember if a hub type has been mentioned the correct advice must include both possibilities.

Do know that the durability of the plastic protectors is pretty poor. The tabs, be they to fit over freewheel threads or onto spokes of a cassette design, get dried out over time and embrittle, cracking/breaking often. Then the protector flops about on the hub sounding pretty bad. Also even with a protector the chain can still manage to do damage to spokes sometimes. The spoke tabbed protectors don't cover the spoke heads. Ask that shop where along the spoke the chain damage usually is and they should say right at the head/elbow.

Can the bike be fitted with a shorter caged der and still handle the gearing range? Andy