French Napoleonic regiments typically had anywhere between 2 and 7 battalions. The 5th battalion was the depot unit with 4 fusilier or in legere regiments chasseur companies. The senior battalions had 4 centre fusilier if chasseur companies each supposedly of 140 all ranks, plus one grenadier (carabinier for legere) and 1 voltigeur company. The 21eme de Ligne for example when part of 1e Corps in 1812 set off with its four senior battalions the first carrying the regimental eagle, plus a new 6th battalion formed with six fusilier companies with an officer cadre from the Imperial Guard and new subleturns direct from St Cyr. In addition to all of that you can throw in an artillery company equipped with 4 Austrian 3pdr guns. At the same time the 7th Royal Fusiliers in the British army had 4 battalions each of 8 fusilier companies obstensibly each 100 strong plus one light and one grenadier company same size, 4th being recruited from Yorkshire. Each battalion served in different places, totally separate from the other. Other British regiments had anywhere from only one battalion to around 8 that last being the 60th Foot (Royal Americans) and them later becoming the KRRC.
In 1881 Lord Cardwell reformed the regiments giving each its own recruiting district, it's own affiliated militia and volunteers. So the Duke of Wellingtons regiment took the old 33rd (1st Yorkshire, West Riding) and added them to the 76th(Hindoostan) Foot, as the third battalion they got one of the West Riding Militia regiments and then for the 4th and 5th battalions around a half dozen rifle volunteer corps. He also at the same time regularised battalion rolls and what have you.