How to determine if my portfolio overleveraged?

[Chad Kastel]

I understand the concept of being overleveraged with-in an individual property.  My question is about if my portfolio is overlevereged?  Generally speaking my plan is to buy properties that are under the market value and must cash flow at least $200 per month.  I'm about to close on my first rental property. I am planning on Brrring  and then to rinse and repeat and rent out the properties for several years.  

How much debt  can I can on before I'm overleveraged (lets assume that I don't lose sleep at night over this and try to make this about the numbers)?  How much cash reserve should I have?   Should I have enough for 6 months? One year? Two Years?  Should this number grow as the number of properties I have?

I am trying to base this information on a crash in the late 2000's.  If something like this happened, how do I predict my vacancy percentage?   Do I assume it goes to 60%?  50% 20%?  

By knowing the worst case scenario for this information I can better determine if I'm taking on too much risk and can adequately take on cash reserves and plan for the worse.