One common instrument for counting vesicle-sized particles is nanoparticle tracking analysis, or NTA. It follows light scattered by particles moving in a liquid and reports how many it sees in a size window.
NTA counts particles. It can’t tell a vesicle from a protein aggregate, a lipoprotein or, in milk products, a casein micelle of the same size. On its own, an NTA figure describes particles in a size range, in that sample, on that day.
MISEV2023 asks for more: an estimate of what else came along with the vesicles, the detection limit of the method, and a second, independent measurement. Paired with electron microscopy or a marker test, the same count starts to mean something specific.
When you see a count, ask which instrument produced it and whether it was measured on the finished product or the raw ingredient.