What countries are you talking about specifically – the main protagonists – Sweden and Russia?
It depends on the theatre of operations, army and time period. In general, the Swedes didn't operate with full regiments, although you will see 2 battalions of a regiment in the orbats of various battles – especially those with Karl XII as commander (the 'Crown' or 'King's' Army). If you look at the orbat for Helsingborg or Gadesbusch – most of the regiments contain 1 battalion or 2 very understrength battalions (aka – the 'regiment').
In Finland, most of the regiments were scattered throughout the lower half of the country (for the Indelta regiments). If you examine the orbats for the battles in Finland, most regiments fielded 1 battalion that was understrength (300-400 men). The garrison regiments were even worse.
For the Russians – in the earlier stages of the war – 1700-1705 – the Russian regiments fielded mostly one battalion, although the 2 'Guard' regiments could field both battalions, as Peter wanted them to gain experience – especially in the battles and sieges in the Baltic theatre. However, according to most reports – the Russians fielded 1 battalion. For instance, Swedish spy reports in 1707 (those spies were reporting to the Swedish commander Lybecker), stated that most Russian regiments fielded 1 battalion of 700 men or less. If you want the exact quote, let me know, as I'll have to look for it in the library.
As with most armies through the ages, 'regiments' in the GNW were woefully understrength – disease being the main killer. Therefore, you could have a 'paper' strength regiment of 1000 men fielding only 400 men divided into 2 'battalions' of 4 companies each (average of 50 men per company). This was the average in the Finnish campaigns later in the GNW.
If you have a copy of "Karl XII pa slagfaltet" – it gives you a pretty good idea of how the Swedes, Saxons and Russians operated. Furthermore, you can pick up a copy of the recently published "Carl Gustav Armfelt and the Struggle for Finland During the Great Northern War" by Helion. This book is a translation of THE book on the GNW in the Finnish theatre of operations. Here's the link to it on the Helion website:
link
In general, I'd stick to the 1 battalion = 600 men rule, as that seems to work best for figuring out pike to musket ratios among the Russians and Swedes. The Saxons, Danes, Prussians and Holstein-Gottorpers would have followed a similar organization. So, a regiment would average 1200 men. Yes, regiments raised in Finland normally averaged 1000 men on paper (divided into 2 'battalions' of 500), but to make it easier when calculating melee – try to keep your battalions the same strength, i.e. average of 600 men (24 minis in my 15mm armies).
Hope this helps. Oh, if you want to know the resources I used, just drop me an email:
jestarnes66@gmail.com
Merry Christmas!