In a two-stroke trials bike, we generally use 15ml of two-stroke oil to 1l of petrol (i.e. in a 5 liter fuel can, add 75ml of oil, and in a 10l can add 150ml.
Follow the instructions in the bike's User Manual.
Also, be aware that not all two-stroke oils can operate at such low concentrations. For example, Castrol Power1 Racing synthetic oil is not rated at such low concentrations. I use Ravenol synthetic. Be especially wary of cheap oils, but check whether an expensive oil is rated at 1:66 mix ratios.