Two kinds of reserves
Search for reserves for a rental property loan and half of what comes back is about landlord reserves: money you set aside for repairs, vacancy and capital expenses. That is good practice, but it is not what a lender is asking about.
Lender reserves are a qualifying test. The question is whether, after you have paid the down payment and closing costs, you would still have enough liquid assets to cover the payment for a period if the rent stopped.