How would I count these groupings to a metronome? What is the correct way to interpret groupings such as these? Note: Time signature for this is 12/8.
This will not be expected to be played in exact measure. Get a metronome and work out which beat each new run/phrase begins and then play all the notes in that 'run' before the next phrase is due to begin and you will automatically be playing in time.
This is just a bit of 'widdley'. Don't worry about precise counting. Get the notes in, with impressive speed and attitude!
Well if you actually do want to count that and play it accurately, you'll need to divide the count up. The 10-plet in the start of your example could for example be split into a 6+4 count, from looking at what you play there, or even 2+2+2+4. Also if you have no experience with quintuplets, septuplets and so on, learn those first to get a feel for larger tuplets.