I believe UAL has ~235 airframes on order or acquiring through leases (319). Assuming 100 airframes are replacement aircraft:

747 being replaced by 350
757 replaced by max9
767 replaced by 787

UAL is still shrinking express and growing mainline. Not all orders are replacement but I would imagine we are headed (assuming a lot of things don't happen) towards 850 airframes. The biggest difference between us and delta is domestic revenue. Delta received the 717 at the perfect time and allowed them to kill it on the domestic side. UAL is not blind and will try to capture as much as they can by upgauging and eliminating the 50 seaters. 15000 seems like the number the company is willing to repeat over and over.

That's my best guess. An additional 300-400 pilots above attrition over the next 5 years.