There is no hard and fast rule for this. If the Examiner discovers any major deficiencies in you knowledge and/or skills, that will lead to failure of that Section of the Test. Failure of more than one section requires the whole Test to be repeated.
Minor deficiencies are entirely down to the discretion of the Examiner.
A read through the Guidance for Candidates, here:
https://www.caa.co.uk/docs/33/srg_lt..._V7_Sept12.pdf
might be worthwhile.
Although it does give a very formal and rigid impression of the Test, Examiners try their best, whilst remaining within the spirit and rules of the Test, to make it as stress free as they can.
MJ
