Can you expand the question. Like, in custom audience you have again multiple options. Website traffic, customer list, video views, etc.
For customer list generation, yoh need atleast 100. For others nothing.
We don’t have to choose audience size for lookalike audience, here we can select percentage from 1% to 10%.
Reach and frequency depends on a number of factors.
You could parse an audience of 1M+ down to less than a dozen due to a number of other “targeting” variables.
The more restrictive you try to be, the less likely whatever it is you are trying to do is going to “work”. How often is that one person logged into FB and actively engaging with content that could potentially generate an impressions for you?