1. You will need to tag each signer as white text. You can choose black for placement first for testing but then you will need to edit the pdf and change font color to white so it won't appear in the final document.
2. You will need to create a duplicate merge document for different number of signers. HelloSign will error out if you have a document with two different signers but only send one signer. Example, if your document has [sig|required|signer1] and [sig|required|signer2]. You MUST choose two owners to send over for signatures. This requires you to have multiple documents in MCA Suite. However you can always use Document Merge categories to help you filter and choose the right document.