Reached cloud router limits

There is a hard limit of Cloud Routers per VPC/Region and is 5. It is not possible to increase this limit

If you are planning to choose another region to deploy your Cloud Router the considerations depends your use case, it’s recommended to choose a region and zone closer to your point of service in order to decrease the latency, also I kindly suggest to review the Performance Dashboard in the Network Intelligence Center menu, that gives you visibility into the performance of the entire Google Cloud network, as well as to the performance of your project's resources. For pricing, Cloud Router is free of charge although general networking costs apply to control plane (BGP) traffic

About your last question, for normal operations, in one region, one Cloud Router is enough, a single Cloud Router can handle up to 128 BGP sessions, with 1 BGP session over each vpn tunnel or interconnect. All the routers in GCP across all regions should ideally be in the same ASN