Greetings!
What you are running in to is the system defined limitation for the level of service you have subscribed to (details here: Transaction Limits)
Because this is a service level limitation, I do not believe that Support will be able to adjust this value for you.
Signing up for a Trial account can give you temporary relief from the limit, as trial accounts are patterned after a higher level of service.
However if this is an enduring pattern to how your transactions need to be configured, you ultimately will need to sign-up for at least an Individual level of service that provides for 10 signers per transaction.