Metric oil seals are more or less straight forward as the size is stamped on the seal. for example 20 47 7 indicates that the inside diameter is 20mm, OD is 47mm & width 7mm. Many seals have different lip arrangements.
SC=R21 & TC=R23 being the most common R21=Single Lip R23=Double Lip
So for example if you had on your oil seal TC 20 47 7 you would now know that you would need a double lip seal with a 20mm inside, 47mm outside & 7mm width.