diff --git a/vllm/entrypoints/cli/serve.py b/vllm/entrypoints/cli/serve.py
index f06a93913..8dfa19e16 100644
--- a/vllm/entrypoints/cli/serve.py
+++ b/vllm/entrypoints/cli/serve.py
@@ -50,14 +50,65 @@ class ServeSubcommand(CLISubcommand):
         if hasattr(args, "model_tag") and args.model_tag is not None:
             args.model = args.model_tag
 
-        if args.headless or args.api_server_count < 1:
-            run_headless(args)
-        else:
-            if args.api_server_count > 1:
-                run_multi_api_server(args)
+        if args.headless:
+            if args.api_server_count is not None and args.api_server_count > 0:
+                raise ValueError(
+                    f"--api-server-count={args.api_server_count} cannot be "
+                    "used with --headless (no API servers are started in "
+                    "headless mode)."
+                )
+            # Default to 0 in headless mode (no API servers)
+            args.api_server_count = 0
+
+        # Detect LB mode for defaulting api_server_count.
+        # External LB: --data-parallel-external-lb or --data-parallel-rank
+        # Hybrid LB: --data-parallel-hybrid-lb or --data-parallel-start-rank
+        is_external_lb = (
+            args.data_parallel_external_lb or args.data_parallel_rank is not None
+        )
+        is_hybrid_lb = (
+            args.data_parallel_hybrid_lb or args.data_parallel_start_rank is not None
+        )
+
+        if is_external_lb and is_hybrid_lb:
+            raise ValueError(
+                "Cannot use both external and hybrid data parallel load "
+                "balancing modes. External LB is enabled via "
+                "--data-parallel-external-lb or --data-parallel-rank. "
+                "Hybrid LB is enabled via --data-parallel-hybrid-lb or "
+                "--data-parallel-start-rank. Use one mode or the other."
+            )
+
+        # Default api_server_count if not explicitly set.
+        # - External LB: Leave as 1 (external LB handles distribution)
+        # - Hybrid LB: Use local DP size (internal LB for local ranks only)
+        # - Internal LB: Use full DP size
+        if args.api_server_count is None:
+            if is_external_lb:
+                args.api_server_count = 1
+            elif is_hybrid_lb:
+                args.api_server_count = args.data_parallel_size_local or 1
+                if args.api_server_count > 1:
+                    logger.info(
+                        "Defaulting api_server_count to data_parallel_size_local "
+                        "(%d) for hybrid LB mode.",
+                        args.api_server_count,
+                    )
             else:
-                # Single API server (this process).
-                uvloop.run(run_server(args))
+                args.api_server_count = args.data_parallel_size
+                if args.api_server_count > 1:
+                    logger.info(
+                        "Defaulting api_server_count to data_parallel_size (%d).",
+                        args.api_server_count,
+                    )
+
+        if args.api_server_count < 1:
+            run_headless(args)
+        elif args.api_server_count > 1:
+            run_multi_api_server(args)
+        else:
+            # Single API server (this process).
+            uvloop.run(run_server(args))
 
     def validate(self, args: argparse.Namespace) -> None:
         validate_parsed_serve_args(args)
diff --git a/vllm/entrypoints/openai/cli_args.py b/vllm/entrypoints/openai/cli_args.py
index bafa845cc..fe874f88d 100644
--- a/vllm/entrypoints/openai/cli_args.py
+++ b/vllm/entrypoints/openai/cli_args.py
@@ -283,8 +283,9 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
         "--api-server-count",
         "-asc",
         type=int,
-        default=1,
-        help="How many API server processes to run.",
+        default=None,
+        help="How many API server processes to run. "
+        "Defaults to data_parallel_size if not specified.",
     )
     parser.add_argument(
         "--config",