How do you have multiple mortgages without over leveraging?

[RZ Holli]

Im an uber newb still absorbing. My current question is "How do you have multiple mortgages without over leveraging?" If I want to hit 90 units and enjoy it I need to build a portfolio with several units a year.  

When do I know its okay to get another unit? 

Do I need a war chest for CapEx and vacancy before picking up the unit it?

Thanks

RZ