I'm trying to say "it counts the number of times a given value repeats sequentially" in a list, meaning it finds the maximum number of times a value occurs in a row in a list. I know there's a word for it, but I can't remember it.
Repeats sequentially is the wrong expression, as OP suspects. The normal phrasing would be...
It counts the number of times a given value appears consecutively.
How about: '(Occurs) one after another'?
… or, perhaps the word you're looking for is 'consecutively'?
EDIT: as FumbleFingers beat me to it, here's something more elaborate (perhaps not what you want):
"It shows the quantity a given value repeats itself consecutively".