Depending on the type of construction, and local codes may be different, at a minimum you should have 1 sq.ft of vent opening area for every 300 sq.ft of attic floor area. More is better.
If vents are inadequate or blocked, moisture vapor can't escape readily enough and can condense on cold surfaces.
One thing to look at is it's possible there is heated air leakage into the attic from the living space.
Check any plumbing vent or chimney pipe chases for a good seal. If you have pull down attic stairs, make sure it's properly weatherstripped.--
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