There are many subtly different styles of playing hosho with mbira. When you hear a short crisp sound followed by a swoosh, the first short sound is the primary beat. When you hear 3 sounds, the primary beat can be the 2nd or 3rd sound, depending on the style of the hosho player. But if there is a swoosh (there isn’t always), then it is after the primary beat.
I would add that the concept of one beat is Western, and all the hosho sounds are important in the music, and may have dance steps that fall on them. But what I am calling the primary beat is the one that people hearing mbira are most likely to clap on, if clapping in a simple way. More complicated makwa (clapping) with mbira is another thing with much rhythmic variety.