Adobe Sign requires the Plan Adobe Enterprise.
Refer the Adobe Sign article Adobe Sign plans and pricing to find out more about the pricing plans of Adobe Sign.
The integration that Adobe Sign has with Xero is separate. Due to their special arrangement, Adobe require a separate account for FYI and with other integrated app partners.
DocuSign requires the Business Pro plan (at minimum).
Refer to the DocuSign article eSignature Plans and Pricing to find out more about the pricing of DocuSign plans.
How many Subscriptions are required - one each or one per practice?
With either Adobe Sign or DocuSign, a single subscription is all that is required to run e-signatures in FYI.
For Adobe Sign we also provide the option for you to have separate accounts for separate users. Refer to Connect your Digital Signature Account to FYI in Digital Signatures (using DocuSign or Adobe Sign).