Hi Trần,

I have no personal experience with contests with 500 teams, but during some onsite contests (like the ICPC World Finals) we have run DOMjudge for ~125 teams on a few laptops.
The DOMjudge webserver and MySQL were running on one laptop (I believe it was a ‘recent’ Intel Core i5 or i7). We had a backup laptop with MySQL replication set up in the case the primary would go down.
We had around 8-10 judgehost laptops (with the same hardware as the DOMjudge webserver), but this highly depends on the number of testcases and the duration per testcase. Make sure you know these variables and you should be able to calculate how many judgehosts you need.

For 500 teams I think one laptop might not be enough, but either a more powerful machine or several decent machines will work.
Note that if you want to have more than one DOMjudge webserver, you need to share the sessions between these machines.
We have also created a wiki page to help you with the setup of these session as well as some guidelines as how to test the performance: https://github.com/DOMjudge/domjudge/wiki/Scaling-and-load-testing

Hope it helps!