Think it only depend on the memory size, but I have heaps more then 50,000 extra POI's and heaps of room left yet. Just be careful as to how you load them as you will have the GPS beeping every 20 metres.
John
I posted this once before but if you want to be sure you have the maximum amount of room for POI's you can gain a substantial amount of room by deleting all the voice/text/help files in other languages. They can be added back in later if you need to do so.