8%. The fraction of AE coins of Constantine with helmeted busts. 

As of Jan. 22, 2025, a search of acsearch on
Constantine 337 helmet*
got 3649 hits. The inclusion of "337" was to eliminate coins of other emperors named "Constantine". A glance through the hits showed most were appropriate. Some were not, for example, some were helmeted CONSTANTINOPOLS types and some types were from the Constantinian era but not helmeted.
A search on
Constantine 337
got 43262 hits.
3649/43262 = .084. So, about 8% of the coins of Constantine I had helmeted busts. 

Go back to the page on Roman imperial coins with helmeted busts.