Slots cardinality on product creation form

Hi and thanks to the authors for this great module.

My use case : some users (role based) are able to create new products holding a registration field (e.g, a user with the "training center" role can create a new training session, which is a product. Users with the "student" role can buy a seat for this training session). So they have access to my product display creation form, with the inline entity form for the product reference field. Here they have a drop down to select the registration type.

It's working fine so far, but now I need to allow the training center to set the number of slots available for each session (i.e each product) they create. I'm not really sure how simple this is, since - in my understanding - it is a field parameter. Since the number of slots available is never the same, creating multiples registration types (for each cardinality) is not an option.

Is it doable out of the box or do I have to tweak something in code (like altering a form or something) ?

Sorry for my poor english :), I hope it makes sense...