Windows Recovery USB

If they are all the same model and were originally shipped with the same/similar configuration, then one should be fine, especially if you have the "backup system files to recovery drive" option checked.

If they are different models, you can still use one recovery drive for basic troubleshooting, but you won't be able to recover the system to factory (if the backup system files option is checked upon creation) unless you're using the correct model that it was designed for. In that case, you should create a recovery drive for each system.