Obviously it's a 50% chance for every roll. So if you want to guess low 8 times in a row the probability would be;
0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 = 0,00390625 = 0,4% chance more or less.
It doesn't matter what you guess, it will never he more or less unlikely for your guess to happen than any other specific guess.
H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H/H
Is as likely as;
H/L/L/H/L/H/H/L/L/H/L/H/H/H/L/H/L/L/H/L/L/L/H/L/L/H/H/L/H/H/L/H