It's in the high 80's in Tulsa today, so the venue must surely have its A/C on. With 20,000 people indoors, even in the evening, it will be fairly uncomfortable without a little ventilation. If anyone is infected (very hard to know which models to use to predict that number, as people are coming from all over), they will be breathing out about 100,000 virions with each breath while not talking or yelling. This is enough to infect 2 people on either side of that person within about 10 minutes. Within 20 minutes, it's enough to infect a couple of people in front of that person.
We know that CoVid has spread on at least two main east-west highway arteries in the US - rather dramatically related to travel stops, convenience stores, etc. So it is likely that at least 4-5 people in that crowd are actively transmitting (and may not know it yet - highest transmission is just before the more serious symptoms occur (or in the case of the asymptomatic, from day 3-14 of infection, approximately). We don't really know how long people-in-general stay infectious, the average is probably about 2 weeks after the fever spikes, but we know very little about the asymptomatic or paucisymptomatic (my new word of the month - few symptoms).
We know some people are still transmitting virus 21 days out and I think the record is 28 days.
If someone is coughing and has CoVid, they will likely infect 5-10 people around them.
Of people who get CoVid in the US, 5% will die. 54 and under - only 1%. Obviously, older age groups are more vulnerable, with over 90's being in the range of 10-20% mortality.
Looking at the crowds in Tulsa, I can see that many are obese and many are older. They're already pressed together outside, where transmission should be lower, especially if there's a breeze and it is sunny. But again, if anyone is coughing or sneezing, the people nearby can still get it.
This becomes a natural experiment for all of us to watch. Many of the people are not from Tulsa, they will go back to where ever they go back to. Some of those places will not be doing contact tracing, etc., so we will never know for sure what the impact of such an event will be, but I predict that some of the people who work at that venue will end up infected, be hospitalized and the public told about their situation.
BTW, even a 1% risk of dying of CoVid (if infected) is way beyond my personal risk allowance. When I was on a medicine that had a 1 in 120,000 risk of causing blood clots, I thought that was not too bad. 4 weeks later, I was unconscious on my kitchen floor. But 1 in 100 is much, much scarier.
I hope people who are taking that kind of risk in a big indoor venue (of whatever kind) have their affairs in order, just in case. Especially a medical directive, because patients with CoVid, if hospitalized, really do need an advocate.