Hi Kerri, and welcome aboard! (Pun intended.

)
You are talking about RoboSource Control 3, right? (As
opposed to the version that came with RoboHelp X5.) From what I've
seen so far, you aren't likely to run into trouble until you get
into the 1500-2500 topic range. We have a few projects of that size
and run into problems now and then. To be fair, the problems are as
much on the RoboHelp side as on the source control side.
There isn't really a hard and fast limit on size, as project
structure and complexity plays a big role. (Your mileage may vary.)
G