How much should we keep in reserves?

[William Lackey]

Is there a certain % (or rule of thumb) I should keep for reserve funds? 

Wife and I just bought a duplex. We put 25% down. Our expenses on the property are $705 a month. We will be renting both sides for $900 each- $1800 total. How much money should we keep set aside? Should we view each unit separately or just view the duplex as one property since it’s under the same roof? Thank you in advance BP fam for helping some rookies!