I was reading about some of the Scotch Brigades/Units/Regiments that were fighting in the conquest of India priior to returning to Europe and fighting against Napolean, and I was wondering how did regiments get such numbers? Why woud the 74th be called the 74th? For that matter, how has that affected modern naming/labeling? WHich leads me to why was the 101st Airborne called the 101st?
For the British regiments it was in order of their raising, so the Royal Scots (raised 1633) were the 1st regiment of foot , the 2nd was what later became the Queen's Royal Regiment (raised 1661) and so on. The numbers were only introduced in 1747, before that regiments were named after their colonels, so what might be "Smith's Regiment" one year would be called "Jones' Regiment" the next if the colonel changed- confusing for all concerned.
The County titles (Northamptonshire Regiment. etc) that you'll find in histories of WW1 and 2 came later by the way, a Victorian-era reform.