spez. This may be fake news. It seems like the interval between shots doesn't really matter and it's the total number of shots that matters. Also the total number of artillery guns doesn't matter either as long as 21 total shots are made. Presidents get 21 shots while others will get lesser number. For example, VPs or foreign dignitaries get 19 shots.
sauce. https://en.wikipedia.org/wiki/21-gun_salute#United_States
https://twitter.com/reaHJFarnsworth/status/1353482161962115072
https://www.youtube.com/watch?v=xiNAly9CKSs - Biden, seems like 3 guns and 10 second intervals is correct.
https://www.youtube.com/watch?v=eFX_7IR91Vk - Trump, 4 guns and 3 second interval
I didn't count in the video but the commentator says it's a "21 gun salute" so I'll just take his word for it.
https://www.youtube.com/watch?v=ZwvbQR887W0&t=8h12m15s
It's honestly impossible to tell because it is not one continuous shot of the guns shooting but I think I counted around 20 shots and maybe there was 1 I missed.