Question about what Just Dance versions to buy

Hi,

I recently purchased Just Dance 2015 for Xbox One and like it. I'm a little confused about what to purchase next though.

If I understand correctly a copy of Just Dance 2017 plus Just Dance Unlimited would give me access to ALL of the songs included in the 2015 and 2016 versions? Meaning there is no reason to own anything besides Just Dance 2017 as long as I pay for the subscription? How many songs would I be missing out on by just buying 2015, 2016, and 2017 and not signing up for Unlimited?

Thanks!