You can get a certificate for any servers you own, there's no legal problem with that. A certificate is just a file, after all. You can use the same certificate on any port, it does not matter.
If you have a certificate for *.you.io (called a wildcard certificate), you can get a bunch of VPS's (e.g. vps1.you.io, vps2.you.io, vps928.you.io), on any provider you want (Azure, AWS, DigitalOcean), put the same certificate on them, and they will be valid, whatever the port you choose.
If one registrar does not allow you do use the same certificate on more than one server or port, buy the certificate from another one. There's no technical reason not to allow that, other than making you pay twice.
And as Liam said, Let's Encrypt is issuing everyone certificates for free, so go there and get one. And they plan to start issuing wildcard certificates later this year, so one certificate will suffice to protect any server in your domain.
All my domains are from them. Except for the fact that they currently expire in 60 days, they work as good as any premium advanced enterprise TLS certificate.