When you visit Control Panel -> Networking -> PTR Records, the only records that will show are those that link the public IPv4 IP address to a Droplet.
You can't modify your PTR records directly as these are setup when each Droplet is created. You can setup multiple Droplets to gain PTR records for each one though.
For example, using domain.com as the domain.
If I setup 4x Droplets:
The above will have a single PTR each. We could change that up a bit as well as use something such as:
And as above, the PTR records will reflect that.
You would need to make sure you have proper WildCard entries setup for the second example though and that would be something such as:
A * DROPLET_IP
A *.web DROPLET_IP
A @ DROPLET_IP
The first entry enables
*.domain.com while the second enables
To handle the requests for those DNS records, you'd need to make sure your web server is setup to handle the WildCard.
For example, with NGINX, we'd use something like:
server_name domain.com *.domain.com;
server_name web.domain.com *.web.domain.com;
On Apache, we'd use something such as: