Martin is right. German, French & British 2 MG per Bn BUT not necessarily all assigned to one Bn. British rarely shared their MG with another Bn or massed those from a brigade. French & Germans seem to vary quite a lot and it isn’t always clear from descriptions of actions how often all 6 MG were massed either in support of an attack or in defence. All that can be said, I think, is that both practices are possible.
Belgians didn’t have enough MMG to field as many as their big cousins but did have small numbers of a light MG which proved very effective.
Russians were short of MG and the training to use them effectively in supporting attacks but they did use them very effectively in defence. It is also said that German reserve regiments, particularly in the east, had less MG in the early days of the war.