Under the new storage model Storage Changes 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.