tunnel_count - (Optional) Number of public tunnels. Required with both Spoke and Transit to be insane mode enabled and max performance enabled. Type: Integer. Valid Range: 1-49. Available as of provider version R3.1.3+.
This is already in the master branch, just not yet released. Will be part of next release. Feel free to test directly against master and report any issues.
https://registry.terraform.io/providers/AviatrixSystems/aviatrix/latest/docs/resources/aviatrix_spoke_transit_attachment
tunnel_count - (Optional) Number of public tunnels. Required with both Spoke and Transit to be insane mode enabled and max performance enabled. Type: Integer. Valid Range: 1-49. Available as of provider version R3.1.3+.