Deberian extender el modelo de subscripción para que al momento de generar una factura ustedes puedan agregar a la subcripción un campo integer para que establezcan ahi la cantidad de usuarios, y otro campo monetary para el monto por usuario y al momento de generarse la factura que se haga el producto de usuario * monto y se actualice la factura.
pero deberán llamara super () y extender el modelo para esto. porque odoo no lo gestiona.

I have also a Software as a service app, same case as Andrew K Lukes. Variable users and also charging per orders.

I had the option in Odoo v15 to generate draft invoices from suscription App. I changed the amount and confirm and sent the invoices.

It has changed in Odoo v16. Not possible to generate draft invoices to update before confirm and sent. I have been investigating in this forum and also in twitter and github but could not find solution yet. I have tried to set products as consumable and to quantity ordered but no invoice in this case. I also installed the app Inventory as I read somewhere but no solution yet.

Is there anybody else with this problem? Solutions?