As with all things cannabis, start low and work your way up
Cannabis affects everyone differently, so it's hard to recommend anything specific, best to figure out what works for you specifically.
So, to be very cautious, try 1ml to get a baseline - if you feel nothing (give it a good while before you decide), then try 5ml and incrementally increase the dosage until the desired effect is reached. It can be a tedious process, but it really is the only way to know what works for you.
Alternatively, you can throw caution to the wind and just take a chug from the bottle - some like a bit of risk to it
And finally, you can follow the recommended dose of 20ml and see what it gets you.
For me, 10ml gave me to a nice relaxed buzz, which was what I wanted from this.