The capacity question decides whether Martyn's Law applies to your site and which tier it falls in, so it's worth getting right.
The question
"Up to how many individuals, including staff, is it reasonable to expect to be present on the premises from time to time?"
What "reasonable to expect" means
It's about your realistic busiest, not your fire-safety maximum and not an ordinary quiet day. Think of your best-attended regular occasion: a Saturday afternoon, a service, a match day, an end-of-term event. Include staff, volunteers and contractors as well as visitors.
Counting
Use what you already have: ticket sales, booking systems, footfall counters, till counts, headcounts at events. If you have several sources, use the highest regular figure. Peak days that happen a few times a year count if they're a normal part of how the premises is used.
The thresholds
Under 200: Martyn's Law does not apply. You can still use PROVA as good practice.
200 to 799: Standard Tier.
800 and over: Enhanced Tier, unless the premises is a school or place of worship, which stay on Standard Tier.
If you're near a threshold
Answer honestly with your best evidence. Being slightly over a line is not a problem; being unable to show how you reached your figure is. Keep a note of how you counted, since it's useful evidence later.
Changing your answer
Retake the assessment if your realistic capacity changes. See "The assessment: what to expect".