What this means in practice
Before publishing several projects, compare their number with the plan allowance.
Domains may remain available even when hosting is not part of the plan.
Practical guidance
Hosting capacity is determined by the plan: Go and Starter include one host each, Enterprise includes five, and Free does not include hosting. A host is a deployment resource, so plan capacity by the projects you need to run rather than by the number of domains alone. A custom domain can point to a deployment, but it does not create another included host.
Before creating a project, check the current host count and whether an existing deployment is still in use. If you have several sites, consider which ones need separate hosts and which can share a project structure. Enterprise's five-host allowance is suited to accounts with multiple distinct deployments. Confirm plan eligibility in the account before building around a host limit; that avoids discovering a capacity constraint at the moment you are ready to deploy.