Your contract may identify a specific number of transactions for each billing cycle.
Billed transactions are calculated as follows:
Sales Orders (minus cancelled orders) + Purchase Orders (minus cancelled POs) + Returns (minus cancelled returns)
You can easily see your Sales Orders by going to Sales Orders > Search and selecting the relevant time range. Once you have your full list of transactions, you can use the Order Status filter to select cancelled orders and subtract that number to reach your Sales Orders billable transactions.
In a similar fashion, you can repeat this process with Orders > Search Customer Returns, and Vendors > Search Purchase Orders to get your Returns billable transactions and your Purchase Orders billable transactions.