Each subscription plan in Katana (except Enterprise) contains a different volume limit which is calculated based on the amount of shipped sales order lines created per month.
At the end of each calendar month, Katana will run an account status check, and if the limit is reached, the account is automatically assigned to a higher plan to match the limit. If the previous limit isn't exceeded again at the end of the next month, then the user can downgrade their plan back to what it previously was. Read more about it here in the QA section: https://katanamrp.com/pricing/
To calculate how many shipped sales order lines have been created each month (not to be confused with total Sales orders), you can head to the Sell Screen > Sales orders tab > Done table and choose "custom date range" (or last month) from the Picked date column to see the results for a certain month.
Once you've filtered out this list, you can determine your sales order line volume by exporting it, opening this exported document, and then checking the number of rows in the #SO column.
Use the row count number and enter it into the pricing plan calculator to find out which subscription plan it correlates with.
NOTE: The UTC timezone is used to count the beginning/ending of days.