1,200 people, 8.5 out of 10. Excellent gig despite the small number of people.
Cozy Powell about this concert in his tour diary.
Emerson, Lake & Powell performed on this day in 1968 at the San Antonio Convention Center in San Antonio, Texas.

ELPowell were at this time very early in their sole tour of North America. The tour overall produced many good memories from those who attended, as well as some excellent recordings. However, it did not often manage to fill the venues at which the group performed.
Cozy Powell, the newcomer to the ELP family and the drummer in this band, kept a tour diary recording his thoughts of each concert. The 8.5 out of 10 that he gave this concert was the highest he yet scored a show.
There is no known recording of this particular concert.
The band performed at the San Antonio Convention Center, known known as the Henry B. González Convention Center after an area congressman. It was originally built in 1968 for the World’s Fair.

The band may have specifically performed at the Lila Cockrell Theater, which was part of the original 1968 construction. The theater currently seats just over 2,300 people and is part of the larger convention center. Its website can be found here.
Venue Sources