When planning your environments it is useful to know the current releases of D365FO that are supported.
Microsoft publish this information. The schedule for specific releases of D365FO is available at this link:
Note: As of October 2021, customers can pause up to 3 consecutive D365FO updates if required.
This means for instance you could elect to stay on 10.0.21, and not apply releases 10.0.22 through 10.0.24.
But you will have to then move to a newer release than your current 10.0.21 release - once 10.0.25 releases (Becomes Generally Available or "GA").
You must also apply a minimum of 2 D365FO service updates (updates that change the release version e.g. from 10.0.21 to 10.0.22 are called service updates), per year (a rolling 12 month period) to your Sandbox and Production environments.
Sites can of course take more frequent updates than that, but this is the minimum.
Note: within a particular release, Microsoft will release additional Quality updates to that release until the "End of service" date listed for that release in the above link.
After that point has passed, the release is still supported but will not be actively updated. So if you find an issue in that release. You will have to apply a newer release to obtain the fix for that issue.
For Release 10.0.21 for example, that means once the End of Service date of December 10 2021 has passed. That release will not receive any further Quality Updates. You would need to have upgraded/moved off that release by March 18 2022, when 10.0.25 is released.