During the hazard perception test in the UK, you should click each time you see a developing hazard. There is no strict limit on the number of clicks you can make, but clicking excessively or in a pattern (e.g., clicking continuously or randomly) could result in a score of zero for that clip due to suspected cheating.
A safe approach is to click when you first notice a potential hazard and then click again when it starts to develop into something that would require action (such as slowing down or changing direction). It's recommended to avoid over-clicking but also to ensure you don’t miss any developing hazards.
Each clip contains at least one developing hazard, and one of the clips will have two hazards. The highest score you can get per hazard is 5 points, depending on how early you identify it.