Reserves for buying rentals

[Shyam Sundar]

I see some landlords say they own 10 or 20 rentals. I am not sure how they are able to do that - not the loan part as I am aware there are funding structures etc. but the cash reserves part. Don't people hold some cash/reserves/savings whatever you want to call it handy, especially the smaller investors. Is there a rule of thumb in terms of reserves/savings you must hold before buying your next (beyond the down payment) - in effect planning for the worst. Example, if I have 100K liquid and readily accessible and I own 2 properties, I can obviously put 25% down for #3 and rent it out but that leaves with me very little wiggle room. I understand this is personal comfort/risk appetite levels, but I'd like to hear how other smaller investors getting into the REI landlord arena think about this aspect.

Would love to hear insights. 

Thanks!