Cap the number of API servers to 1 when using Elastic EP. (#37466)

Signed-off-by: Sage Moore <sage@neuralmagic.com>

vllm/entrypoints/cli/serve.py

@@ -108,6 +108,15 @@ class ServeSubcommand(CLISubcommand):
                         args.api_server_count,
                     )
 
+        # Elastic EP currently only supports running with at most one API server.
+        if getattr(args, "enable_elastic_ep", False) and args.api_server_count > 1:
+            logger.warning(
+                "Elastic EP only supports running with with at most one API server. "
+                "Capping api_server_count from %d to 1.",
+                args.api_server_count,
+            )
+            args.api_server_count = 1
+
         if args.api_server_count < 1:
             run_headless(args)
         elif args.api_server_count > 1: