With 64-bit aggregates, the number of FlexVols that can be created on a storage controller...

http://www.netapp.com/us/products/storage-systems/fas3200/fas3200-tech-specs.aspx

There is no easily remembered reason why its 500. Some people like to remember that there is one other question on the exam that requires you to remember an arbitrary number and the answer is also 500 (distance of a fiber interconnect at 2Gbps).