Reserves on rentals for buy and hold

[Brandon Turner]

Got some start up money, now I'm about to close on two houses next month both already have tenants in place.  I guess I am kinda a little like what now. I guess what I am asking is how much money do ya'll like to keep on hand before you go on to the next house? DO you let the rent carry you back up just enough to get another 20% down payment. Suggestions??