It all varies but I can give a type of an example. Say we have 20 ladies in the house, we pull shifts from 5am-5pm, 5pm-5am, so late at night or first thing in the morning you can automatically cut that number in half, so you are now down to 10. Out of that 10 who are on shift and supposed to be on the floor, you can assume that 1 slept through the announcements to come to the bar or for the line up, 1 is in the shower, and 1 working out, and one is in a party so you are now down to only 6 out of 20.
If you come in between 1pm-1am we are usually up and dressed so even if we are not on shift we may still be very likely to be on the floor and available. During those times if we have a house of 20 you can assume that 2-3 are in a party and the rest are able to come out to talk with you so you will still have 17-18 ladies ready to please you.
Now as I said those are all hypothetical examples of possibilities, we could have more or less ladies available during that time.