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If you want the best & most detailed - you get the NARC (North American Rail Car) rotary gons. They're not cheap ($37 each) and to my knowledge are only available through PWRS (link below), but they are the best and most detailed coal gons I've seen. Currently they have the CN, CP, BC Rail, Procor, and Sultran versions available. Not sure if Procor ones were used on UP's network, but it's possible. The UP-marked gons and a few other roadnames are due sometime this year, if you want to wait. I have 10 of these, a mix of CN and Procor. They're beautiful cars.
How many depends on how big a layout you plan on running these on. A smaller layout may only be able to handle a smaller 7-15 car train, while on a large club layout you could probably run many dozen. As for DPU - you can always run DPU, even with a smaller train
Back around 2003/2004, the UP coal trains ran by my brother's house in Littleton, CO. As I recall, they were about 112 cars. BNSF and UP used different dpu schemes. I think the UP place them either at 1/3 or 2/3's of the way back in the train, with BNSF placing them at the rear. Hope that helps.
The best coal cars depends on what era and location you model. Unless you don't care and just want the best or most detailed hoppers.
I model D&RGW of the 70's and 80's and really like the Athearn/MDC RTR Hi-side Thrall coal gons and the Ortner 5-bay rapid discharge hoppers. They are accurate for many coal trains D&RGW hauled, and have good detail. As others have noted, ExactRails Bethlehem 3843 4-bay hoppers are very nicely detailed and accurate for D&RGW, BN, CB&Q and GN manufactured in the 60's 70's and 80's. Tangent makes a very nicely detailed 4-bay hopper accurate for UP and a number of other roads.