u16–20 prawns refer to the size of the prawns, that there are approximately 16–20 prawns per pound. The lower the “u” number, the larger the prawns (i.e. “colossal prawns are u6–8)
For Jalapeño puree, remove stems, seeds, and veins from the jalapeños, and place into a food processor until pureed. Be careful not to stick your face into a food processor, the oils from the chili will aromatize and could cause a coughing fit
If you can’t find Thai fish sauce, the Vietnamese counterpart will do the trick
Once the prawns turn white/opaque, they are virtually done, as they will continue to cook from residual heat.
If swapping out for chicken or pork, browning them slightly in a separate pan before adding to the sauce (which will finish their cooking) will result in a tastier end product