Hum.
A single floating identity per two ranks.
That is, if you have 4 ranks, you have two separate mortal identities.
They seem mortal to all but the closest of scrutiny, save if they use divine power.
Donning or leaving one would be a Complex Action.
If you get caught and the identity is compromised, you can make a new one, so long as your total does not exceed your max.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather