Per the Workers Platform Limits docs:
Each Worker invocation can have up to six connections simultaneously waiting for response headers. ... If a seventh connection is attempted while six are already waiting for headers, it is queued until one of the existing connections receives its response headers.
In production this cap is enforced (six in‑flight subrequests). In wrangler dev / workerd locally, it is not: the Worker can have arbitrarily many in‑flight fetches.