On departure from Builth Road in the afternoon, there was ample room for everyone who wanted to board my train, as some people were staying later and planning to take another train.
Going to Builth Road in the morning, there were some people standing from (around) Llandovery onwards, but the loadings weren't as bad as you'd see in Cardiff at rush hour. Boarding further south was no problem: it was actually more than half-empty until after Llanelli!
My "if you feel there's room" was a bit of a disclaimer on my part, as I can't know what stock they'll have available for the service. I think the train was a pair of 150s when I visited, but I couldn't swear to it. It was a few years back now, and my overriding memories of the day are of trying to avoid the media scrum following Prince Charles around the showground, as he'd decided to visit on the same day.