When preparing your document as usual, SignRequest currently allows adding 20 signers on one document. We highly recommend preparing input fields for such documents before sending as otherwise with multiple signers they will not know where to add their signature. In addition, the signers can not be changed after sending of the document so you should be sure that all signers and email addresses are configured correct.
Please be aware that with such a document, if one signer declines, the other signers cannot sign it anymore.
In case you use the bulk send feature you can send the document to up to 1000 signers at once. This, however, will result in up to 1.000 documents each with one signature. Learn more about sending a bulk SignRequest here.