It all depends on your application. There are a few measurements that need to be taken. Here are some in no particular order:
The scope's objective bell outside diameter (no not 40mm or 50mm, the outside diameter including the housing) if you use flip ups add the thickness of the also
The scope's tube diameter (1" or 30mm)
The height of the mount
With these numbers you can figure out how high of a ring you will need to allow the objective to clear the barrel.
Brownell's has a good chart on this
How to Determine the Ring Height You Need
They also have a video using clay to determine ring + mount height:
Determining Scope Ring Height
The top chart does not take into account hammers, charging bars, or other mechanical devices that may require a higher than usual scope mount/ring combo. Some receivers are too short or the scope needs to be set further forward in which case an offset ring is needed.