Yes that's doable, but you'll need to write custom SQL for it. You'd use CB Query Field to query the database for their number of active or I guess past active subscriptions then multiply it by the discount you want to offer per plan (e.g. 5% per plan). You'd then substitute in the query field as the discount value in a CBSubs Promotion.

Yes, they're stored in _cbsubs_payment_items. You can query _cbsubs_payment_baskets for their most recent basket then check its ID against rows in _cbsubs_payment_items. The count of rows in _cbsubs_payment_items where their payment_basket_id matches the id column of _cbsubs_payment_baskets is how many plans they've in their basket.

The below query for example should give you the plan count of the users most recent basket. You may want to add more conditions to ensure it hasn't already been paid, etc.. but should be a starting point.