As i can see here: https://www.digitalocean.com/community/tutorials/an-introduction-to-digitalocean-spaces, yes! You have to pay for each bucket.

From the knowledge base https://www.digitalocean.com/help/pricing-and-billing/general/

How much does Spaces cost?

Spaces costs $5/month and includes 250 GB storage and 1 TB transfer out, shared between all your Spaces. If you use more than that, the additional storage costs just $0.02/GB/month and the additional transfer costs $0.01/GB. Transfer in (uploads to Spaces) is free, and transfer between our NYC regions is also free. If you sign up for a subscription in the middle of a month, that month’s subscription will be prorated.

I had the same question.
After a search I found this Help Article

it says “Spaces will cost $5.00/month for an unlimited number of buckets, which includes 250 GB storage and 1 TB transfer out from Spaces”