I t has changed recently for ocr but i dont know what to, i think the mark that counts is the highest mark in your last two exams so if you get 68, 65, 56 then your grade will drop as the 65 would be discounted.
i doubt thay are limited i'm resitting all my maths P1 M1 ans S1 and i'm doing P2 in the next week! so i doubt it.
Also if you say got a D first time and resat and got a E, then you can resit again but if you Failed then you can pick only between the Fail or the U, so basically you may only choose between the grade u just resat and the previous one.
But resitting costs money so not really advised!
Hope that helps
you only really take 1 resit. then the best out of the two marks is submitted.
my exam board is Edexcel and as far as I am aware we can resit as many times and the highest grade is considered.
For Edexcel, u can sit each unit twice and the highest result counts.