I've popped in and out of this discussion for a couple of weeks...
I have heard several different rules of thumb regarding force ratio vis a vis security mission... Of course it is all METT-TC dependent (especially enemy, troops, and terrain), but...
Cover Guard Screen
Corps DIV Regiment/BDE+ Squadron/BN+
Div Regiment/BDE+ Squadron/BN+ Troop/CO+
BDE Squadron/BN+ Troop/CO+ PLT+

Source FM 17-95