Not as complicated as some think...one branch of the family started numbering the children from I, II, III, IV, V, etc...beginning with the first born boy at the beginning of a century. So for example say on January 21, 2000 Princess XYZ Reuß gives birth to a boy, on January 24, 2000 Princess ABC Reuß also gives birth to a boy.
These are the two first Reuß boys in the XXIst century...what number do they get? The one born on 1/21/00 would therefore be Prince Heinrich I Reuß, while his cousin born three days later would be Prince Heinrich II Reuß...and so on until the last boy born in the XXIst century.
Another branch, which is now extinct, numbered their boys starting from 1 and ending in 50...once they reached 50 they would start with one all over again so you could have two Prince Heinrich I Reuß born in the same century...a bit confusing though.
Eurohistory pulished a study of the Reuß family authored by myself and David mcIntosh last year which includes all the princes and princesses alive today, family trees and history and explanations about the system.
Arturo Beéche