We assist companies in obtaining GSA Schedules. Will we need a unique digital business certificate for each company we help, or, will a single digital business certificate allow us to submit schedules for various companies?

You will not need a unique digital certificate for each company you work with. You can use your digital certificate to work on any offer that you are a negotiator for. However, if your client is going to sign the final documents they will need their own certificate. The digital certificates are assigned on an individual basis and not on the company level. So, each individual will need their own certificate.