According to NBC News, O'Rourke's team said there were about 7,000 people at his rally and mile-long march beforehand, per law enforcement. And Bloomberg News reported that O'Rourke's rally had been estimated at 10,000 to 15,000, according to local police. At various points during his own rally, Trump claimed that 69,000 people had RSVP'd and that fire marshals allowed 10,000 people in with "tens of thousands" still watching outside. But NBC News reported that Trump's rally was at its 6,500 capacity with 10,000 to 12,000 watching outside or in overflow areas, per the local police and fire departments.
From the El Paso Times:
Trump thanked the El Paso Fire Department for getting 10,000 people in the El Paso County Coliseum, before noting the crowd that gathered outside for his rally.
“Tens of thousands of people are watching the screens outside,” Trump said.
However, a Fire Department spokesman said 6,500 people were inside the venue and denied any special accommodations were made. Several thousand, though, were outside the Coliseum, watching the president's speech on big screens. There were no official estimates, though.
An O'Rourke spokesman said about 8,000 attended the counter-rally. However, the El Paso Police Department told the El Paso Times that officers had made no official estimates.