Very few did. The 101st Airborne division (the division depicted in Band of Brothers) was made up of 4 main, much smaller infantry regiments. Each regiment painted a different symbol on their helmets which corresponded with the 4 suits in a deck of cards. They were:

The 506th Parachute Infantry Regiment (Spades)

The 502nd Parachute Infantry Regiment (Hearts)

The 501st Parachute Infantry Regiment (Diamonds)

The 327th Glider Infantry Regiment (Clubs)

Here is the relevant section on Wikipedia

The purpose was a mix of being able to recognize men belonging to different units in the chaos of a combat airborne drop and unit pride.

But really the 506th PIR was less than 1/4 of the strength of what was only 1 out of nearly 100 different divisions. So to say that many troops had these painted on their helmets isn't really accurate.

Also that's not counting men in the field who would have painted or intentionally covered theirs (they were after all white) or were simply given replacement helmets without the insignia while fighting.