The burial service of a family member that I attended at Arlington was a rifle salute. I would have to dig out photos to see how many rifles there were — can’t recall if it was three or four. The video I saw of the Biden ceremony (I believe I saw it on AWK News - don’t know where he got it) was three cannon. And he counted 10 seconds between each fire. Two cannon alternating firing and one cannon in reserve in case of a misfire.
I’ve been to 1/2 a dozen or so military burials over ~40 years and every one I attended it was 7 rifles, 3 shots each. I know that when personnel aren’t available they make adjustments to cycle, but just haven’t seen any where that is the case personally.