Hi.
One licence per seat means that :
A Per-seat license is a software license model based on the number of individual users who have access to a digital service or product.
Which mean that only one person (you) can use the licence and not to be used by other user.
So unity 'Personal' (free version) works with all asset (including PlayMaker) on the asset store (except if stated otherwise on the asset...if there are any)
On the asset store page at the bottom/right of the page, there is an option to set the currency.