I am vexed y an advert that talks about "must do's".
Surely it is "must dos"?
Both are used.
Click on the link Do's and don't's? and scroll down to Minding my p's and q's … dotting my i's, crossing my t's and other do's and don'ts
Here's another view:
"dos and don'ts — Not do's and don't's" Source: JSU Manual of Style and Usage
All the best.