http://www.winterwar.com/M-Line.htm
From the article:
"And if counting concrete bunkers which had machine guns or cannons (thus excluding passive concrete shelters), a total of 48 bunkers, there was one such bunker for each 3 km of front, i.e. roughly 0,35 armed bunkers / km.
(Compare the above figure with the average number of concrete fortifications / km in the following lines;
- some 10 in the Maginot Line, in the weaker part at the Belgian border
- Westwall had at least 15)"
"The sheer volume of work and too small resources resulted in a line that practically lacked depth. Only in the Summa sector on the "Gateway of Karelia" was the back line (2 km from the front-line) of the main defense line with adequate field fortifications. In most places, tactical depth was achieved by locating some front-line unit shelters 200 -300 meters back from the front."
This is from Wikipedia:
"During the war both Finnish and Soviet propaganda considerably exaggerated the extent of the line's fortifications: the former to improve national morale, and the latter to explain their troops' slow progress against Finnish defenses. Consequently, the myth of the "heavily fortified" Mannerheim Line entered official Soviet war history and also some Western sources. However in reality, the vast majority of the Mannerheim Line was comprised merely of trenches and other field fortifications. Bunkers along the line were mostly small and thinly spread out, and the Line had hardly any artillery."