As of Alpha-2 a new option was created to control the 'Intake' (or 'Continuous Intake') of prisoners. Two options are now availible at the creation of a new prison:
If you have the continuous option enabled, the game will behave the same as Alpha-1, prisoners will arrive every morning in various qauntities. However, they will never exceed 24 prisoners per day (Fixed in Alpha-2).
If you have continuous intake disabled then prisoners will only arrive if you have room for them. This becomes extremely handy when you want to focus on creating a big prison without prisoners getting in the way. Only partition cells when ready to receive prisoners, then the next day "X" amount of prisoners will arrive to full your prison to max capacity.