[dice]

A teaspoon of mycorrhizal spores? I think that would be enough
for about 100 plants. I usually go for "smidgen" sprinkled over
the root ball before transplanting (half a pea or so by volume).

I actually add it at the first potting up, and I expect it to
already be growing in/on the roots by the time I transplant
into the garden or a container. (Since I grow winter cover
crops whose roots are mycorhizzal-friendly, it should still
be there in the in-ground beds next spring, but container
plants could still benefit from a fresh application.)