That bottle contains 15ml, with 150mg of CBD
There are 20 drops in one 1ml
So there should be 300 drops in that tincture.
150mgCBD/300drops translates to
0.5mg of CBD per drop.
The science is nowhere near exact, however, dosing guidelines differ for different conditions, e.g severe epilepsy would require over 100mg/day. However, 10mg of CBD (20 drops)—chosen somewhat arbitrarily—is considered a signle serving for most conditions.
I hope this helps!