Compound calculation of GCP GCE SLAs based on number of instances?
99.99% SLA uptime roughly translates to 52.5 minutes a year of downtime (calculator here).  Generally if the service is down, it is down for everything in that geographical area.  So if you had 1000 servers in the same geographical area and that area goes out of service, your downtime would be tied together.

However, most of the "big" cloud providers (e.g. Google) provide their services across different regions and zones.  By spreading your infrastructure out across different areas, you can make your network more resilient to cloud provider outages.