Sitting on the cercanias train now. Arrived Madrid with a 1.5 hour delay. 10am now, could have probably made the 11am train but it was sold out. All regular seats on the next trains were sold out until 3pm. We snagged first class seats on the noon train, paying a premium but glad to get to Seville as soon as possible.
Trains that have reserved seating will sell out.
Yes, we had wondered about them selling out but had gotten advice that there were enough trains that we should be ok...it seems to be a darned if you do and darned if you don't since you never know if you'll have delays. Personally speaking, if I had it to do over again, I would pre-book a little bit later tickets as a safety net.