United had a total of 51 Boeing 747-400s on order, from the original order placed in 1988 if my memory serves me correctly (that's when they placed the largest order on earth -> or at the time it was. 90 B 757s, 23 B767-300ERs, 135 B 737 300/500 models, and at the time 35 B 747-400s). During the early 90s, the number of 747-400s swelled to 51, only to have 7 orders converted to a handful of 777s (Asian crisis opened up additional 777 slots).
I do see United getting more 747-400s, but I think it's waiting for Boeing to get more done w/ the 747x / 500X or whatever they are calling this new programme.
~ ~ ~ ~ pRoFeSsIoNaL hUrRiCaNe DoDgEr ~ ~ ~ ~