GCP VM with more than 8 network interfaces

I post this answer to make solution provided by @John Hanley in the comment section more visible:

Accordingly to the documentation:

The following limits apply to VM instances. Unless otherwise noted, these limits cannot be increased. For quotas relevant to VMs, see Compute Engine quotas.

Limits such as number of network interfaces is a limit of the hypervisor and management software. Those are virtual network interfaces, the underlying hardware has less than eight network interfaces.

You are limited to eight network interfaces which cannot be increased for VMs. If you are developing a firewall, contact your Google account manage to discuss dedicated hardware options. If you are a customer, then you do not have any options that I am aware of. Contact your account manager to confirm what is possible.