Hi Brian. Here are the answers to your questions:
1. Can the scanning tray hold up to 50 pages at one time? Yes, You can load up to 50 sheets of 8.5" x 11" (Letter).
2. Does it scan to email and USB? Yes, you can scan to email or memory device.
3. How much are replacement cartridges? Please visit the online Best Buy store for pricing on ink cartridges. The WF-4830 printer takes Epson T822 and T822XL ink cartridges.
Regards, Bobby - The Epson Team.