it looks like the reference process that you chose (disposal PET bottle) is using "Heat" as a reference flow, which has the unit MJ. The flow property here is "net calorific value".
The other process that you mentioned probably has the flow property "number of items", which means that you can change the target amount of number of items.
So, it always depends on the flow property of the reference flow that you reference process is using. I have not done this specific tutorial but I assume that disposing of 400 bottles (compared to one bottle) uses 400 times the energy, so you could just multiply here. You could also redesign the disposal process and choose a different reference flow with the property "number of items". or you could change the flow properties of the flow you are currently using. If you open the flow, there is a tab called "flow properties".