How strange, normally something like the tray and staple capacity are posted for everyone to see.
I can see a few options:
1. Contact HP sales and see if they can dig up a number for you on the stapler. The stats wont be included on the printer, only on the stapler so make sure you ask them to look in the right place.
2. Conduct experiments to see how much paper will fit in the staper slot.
3. Use a binder clip or some other material rather than a staple to bind your print jobs.
4. Purchase a seperate power stapler.
If I had to guess I would say the stapler can probally fit around 50 pages of normal paper. However, your results and durability will diminish with larger stapling jobs. You would likely be pushing the limits of this little accessory if you consistenly fill the stapler slot. I would assume the stapler is a convenience item and intended for smaller staple jobs.
If you truely need a powerful stapler then it may be easier to purchase a 2ndary power/heavy duty stapler and locate it next to the printer instead. There are some crazy powerful staplers out there. Once you start getting up and around 50 pages you need to start considering if a staple is really the best binding material to use. Paper can stop bullets if it is thick enough so there are natural limits to how effective a stapler can be.
You can get these Heavy duty power staplers for pretty cheap:
160 page - http://www.amazon.com/Swingline-Heavy-Stapler-Sheets-S7039005/dp/B0006HUPU2