The ratio is constant at 10 off deed.

You can get info about your deed from the Settlement → Info at your settlement token, or from right clicking lower left of your character sheet (C, Manage, Settlement, Info).

The ratio formula:

Deed tiles/15 = # of animals

Example: 21×23 is 483 tiles (21*23). Divided by 15 is 32 animals max.

If your ratio falls below 15, your animals will miscarry and become diseased. Animals that miscarry typically die along with the offspring.