Check FMP on a Summery Field and Summary Report types...Defining summary fieldsUse summary fields to calculate values such as subtotals, averages, and grand totals across multiple records. For example, a summary field can display the grand total of all sales in the month of May in a report.Jim...
This assumes, as it should, that you are using a separate record for each asset. You are not actually counting fields, but rather are counting records.
Here's a tutorial on summary reports that you may find useful: Creating Filemaker Pro summary reports--Tutorial