Hi. You have to buy a Plesk license for each VPS(the Plesk license is per IP). There's no per-node licensing option with Plesk. For a volume discount you can contact Plesk sales to get such licensing option.
Remember you are not restricted to buy Plesk licenses only with OVH: You can get them with any Plesk reseller, or directly with Plesk:
https://www.plesk.com/plans-pricing/ . As you are planning to deploy plesk in VPS, you can take advantage of the VPS licenses ("Virtual"), or, if each VPS willl host no more than 10 domains, you can choose the "Web Admin" license.