How many backlogs are acceptable in UK from Indian students?

Backlogs or Gap do affect you candidature but it can be backed by the strong profile and personal statement/Statement of Purpose. You need to explain the reason for your backlogs along with backlog certificate mentioning the backlogs and marks obtained in each subjects. If you had any medical issues you can provide medical certificate along with other documents.

Every university has a certain limit of accepting the backlogs. Generally 15 backlogs are accepted in the UK but it depends on university and course.