The number of units is calculated automatically and is based on the number of units built in the system.
Why do I see e.g. 2.40 units (quantity) on the Invoice?
This can happen sometimes based on the rule mentioned above. Let's pretend you have 12 units on 25th June. The calculation is done: 12 / 30 (days of June) * 6 (period 25-30/6/2020) = 2.4 units for June