Dear @Dani_MB
I hope i can help :)
The License key that you have from your 2018 purchase allows/activates the use of Lay Theme for one domain.
This came with the Carousel addon, however you can use this Addon on as many Lay Theme sites as you wish.
You will need to purchase a second License key to activate Lay 'Theme' on your new website, but you can install the Carousel Addon ( plugin ) like you did with your first website - you do not need to authenticate this Addon.
I hope this helps and have a wonderful day, thank you for thinking of making another Lay Theme website,
If you have any further questions surrounding this, let me know! :)
Best wishes
Richard