Start with volume
Count the new tickets each week and compare with the last few weeks. A rising line means you are growing or something is breaking. A flat line while customers grow means your help center or product is doing its job.
First response time
This is the number customers feel. Look at the median and at the slowest tenth, because the slowest tickets are the ones people remember. If you are the only person answering, check which hours the slow ones arrive in.
Open backlog
Count the tickets still open at the end of the week. If the backlog grows week after week, work arrives faster than you close it, and a hire or a fix to the top question is due.
Satisfaction
If you send a survey, track the share of positive answers and read the written comments. Treat the percentage as a signal, not a score; a handful of answers can swing it.
The top three reasons people write in
Tag each ticket by topic and look at the three biggest. This is the most useful number for a founder because it points straight at what to fix in the product or the help center.
What to leave off
Skip numbers you would not act on: tickets per agent when there is one agent, or hand-offs between teams that don't exist. Add a number only when you can say what you would change if it moved.