Choose a number correlated with the runes you use. Lögr is 21. So 21 or a multiple of. Perthro is 14. So 14 or a multiple. You can do different numbers of vibrations for different runes in the same working. Although it seems to make sense to keep them relatively close to each other. It’s up to you.
I'm not exactly sure the number as it was years ago but it was 30+ for each rune. It was more of experimentation. Did it on the full moon. My pendulum is onyx.