Because Shielded VM is a type of Compute Engine VM, its quotas are identical to those of Compute Engine.
For Shielded VM instances, the quota and limits are based on number of vCPU. For example, a limit of 8 vCPU means you can launch a total of 4 instances of type "n2d-standard-2 (2 vCPU, 8 GB Memory)."
To learn more about Compute Engine resource quotas, including how to check your quota, how to request an increase in quota, and how quota is enforced, see the Compute Engine Resource quotas page.