Under the new storage model https://www.fkbase.info/StorageChanges additional instances can be created free of charge provided you have sufficient disk space available.
You can manage instances in the "old" admin center, (https://port.crm.dynamics.com/G/Instances/InstancePicker.aspx?Redirect=False) (replace crm as appropriate for your region) but as AFAIK you can't create instance in the old admin center - they appear if you have an entitlement to one or purchase instances.
But you can create instances in the Power Platform admin center https://admin.powerplatform.microsoft.com/environments. I did this and selected Production, include a CDS database and allow D365 apps. The environment was created and soon after appeared in the old admin center. I was a little surprised that I could do this because the subscription I was working in is using the "old" storage model which doesn't provide free instances.
But in the old admin center, when I tried to install a solution such as the sales app, I found I wasn't able to and received this message:
You do not have a Dynamics 365 license for this Dynamics 365 instance. Please get a license at portal.office.com
So it seems that a subscription really does have to be on the new storage model before additional free instances will work.