You may want to know how many active customers you have on your database and to look at this statistic over time, to gauge if it is growing. This can be done in segments. 

1. Create a new segment 

2. Click NEW CRITERIA and in tab 1, Events Customers Attended, create a broad PERFORMANCE DATE search to include customers who may have booked well in advance. 

3. In tab 2, Customers Behaviour to Selected Events, choose the dates for the period you want to look at (eg last 2 years) during which customers should have made a transaction

4. In tab 3, Filter By Customer Attributes, select DATA CAPTURE equals data capture, to get rid of records with no data (these may be walk ups and possibly dupes of other records on your database)

5. Save and count