There are no programmed device limits. Since the number of network elements is based on memory, you should be able to add a few thousand hosts. The only problem will be finding screen space for them. One alternative is to split them into multiple maps and use the "subnet" icon to link the maps. However, you will only be able to see one map at a time.
With this said, the WhatsUp line of software was designed for the small to medium sized business, and for maximum performance and scalability many companies start investigating solutions such as HP OpenView once their object list exceeds a thousand.
