Only you know your limits.
One thing that is often forgotten is it's very important to have some cash squirreled away for any unexpected vet visits and/or medication.
And in many cases treating one means treating them all.
And I think it's best to take your time. I've seen more than enough people get "hooked" and add a bunch of animals in a short time period. Them burn out.... before you know it there's another "collection sale" on Craigslist...