Carlos,
You should read our High Density Auditoriums VRD if you haven't seen it.
500 seats is actually quite small as these rooms go.
For this small a space, I recommend using the same client count for both associated users and concurrent users. We advise 50 concurrent users per radio, which is 100 for a dual-radio AP.
So assuming even distribution of 2.4G and 5G clients, you need 5 APs. 5 * 100 = 500 users.
Now as far as capacity, you will want to run HT20 channels. In 5GHz you wont have any channel reuse. Say you are on 149, 153, 157, 161 and 165.
In 2.4GHz you will be reusing 2 channels. Say 1, 6, 11, 1 and 6.
In 5GHz, an HT20 channel does about 80-90Mbps TCP for a 2SS device and 40-50Mbps TCP for a 1SS device. Using 50Mbps as an intermediate value, for 50 users, thats 1Mbps per user.
In 2.4GHz it will be half that on channels 1 and 6 since they are reused (e.g. each channel has 100 users not 50 users).
-cl