That depends on the rifle, and on your barrel profile. Typically, a riflescope with a 50mm objective lens will use high rings.
However...
If you have a bull/target/heavy barrel on that same rifle, extra-high is indicated.
If you Have an AR-style flat top, extra high is used, regardless of the objective lens diameter. Because the barrel is in line with the stock, an optic on an AR must be raised to the height of the original iron sights, or you won't be able to get your eye low enough to see through the optic.
Mark H.