This is not a limit that is set by Gozynta, Twilio sets a limit on the amount of tickets you can have in your system open against your Tixt number.
Every time a Tixt ticket is opened up by a user texting your tixt number, we have to provision a number from Twilio to be used for communication on that ticket. When you get this error message, that means we have reserved so many numbers that there aren't any left for us to get from Twilio.
To clear this error, please start closing out your tickets so they can release the number assigned to the ticket.