IG organizational structure is extremely loose and nonstandardized (for example, you'll find Colonels, Commanders, Marshals, Battle-masters, Tribunes and Warlords in charge of Regiments, Armies, Legions and Battallions, all at about the same rank), which results in a lot of variation. The number of soldiers in an individual regiment varies significantly based on what sort of regiment it is, where it was raised, when it was raised, who raised it and what sort of history it has. Notably, the 597th you use as an example is a mechanized regiment, not infantry, which accounts for the unusually low overall strength. Armoured and artillery regiments are typically even smaller than that, with a typical tank regiment, according to the old Armoured Company rules, consisting of somewhere in the region of 600 soldiers operating and maintaining 95 or so tanks.
Generally speaking, Cheesegear is correct about the sheer variation in infantry regiments, though 100 is seriously low-ball; generally, you won't see a Regiment with less than 500 men. That said, if you want the "gold standard" of infantry regiments, the Cadian VIII is made up of 24 companies, each 300 soldiers strong, plus supporting elements including chimeras and sentinels that bump the regiment's overall strength to about 8000 men.