There's a number of factors at work that are limiting the number of coaches, sleepers, and even sightseer lounge cars on Amtraks trains these days.
1. During the pandemic, Amtrak stored a large number of cars. It's likely that the 90 day inspections expired as well as other regularly, required scheduled inspections and servicing. That means there's a good chunk of money required to get each car 'back on the road' again. $10-20,000 maybe more.
2. Numerous car maintenance and repair workers were similarly laid off during the pandemic. Whether they were called to come back and declined (had found work elsewhere) or decided not to work at Amtrak any more is unknown. From what I've read here and there, new hires can't be 'ready to go' when hired. It's far more complex than hiring an auto mechanic. Finding drug-free, alcohol-free, criminal record-free workers these days is getting harder every day. So there's a shortage of shops staff.
3. Add to that a shortage of on board services (OBS) staff. Many laid off. Fewer wanted to come back to long overnight schedules and irregular work days like 3 on, 2 off, etc.
4. Insufficient T&E staff. The freight railroads are having increasingly more difficulty having sufficient T&E crews. Training isn't a 1-day affair, either. More like months. How many clean and sober new hires are willing to work extra board non-schedules, long, tiring trips, crying babies and whining adults, and a deal with a growing number of beligerent passengers?
5. I don't know what the union contract indicates as number of cars = what number of conductors. But it's quite possible that any lengthening of a train will require an extra assistant conductor as well.
We are all familiar with current Amtrak management 'save first, save last, save always, and Amtrak will prosper' thinking. Put another way, they appear to shy away from doing anything that would generate more income, especially during the summer peak travel season and countless potential passengers are being turned away due to full trains...and overworked staff, especially in the lounge car!