Allowing only one worker in pool (#2656)

* As the title.

* removing f from string

* Adding unit tests

src/ansys/mapdl/core/pool.py

@@ -222,14 +222,14 @@ def __init__(
         self._active = True  # used by pool monitor
 
         n_instances = int(n_instances)
-        if n_instances < 2:
-            raise ValueError("Must request at least 2 instances to create a pool.")
+        if n_instances < 1:
+            raise ValueError("Must request at least 1 instance to create a pool.")
 
         pbar = None
         if wait and progress_bar:
             if not _HAS_TQDM:  # pragma: no cover
                 raise ModuleNotFoundError(
-                    f"To use the keyword argument 'progress_bar', you need to have installed the 'tqdm' package. "
+                    "To use the keyword argument 'progress_bar', you need to have installed the 'tqdm' package. "
                     "To avoid this message you can set 'progress_bar=False'."
                 )
 

tests/test_pool.py

@@ -301,3 +301,28 @@ def myfun(i):
     assert "Other_instance" in dirs_path_pool
 
     pool.exit(block=True)
+
+
+def test_num_instances():
+    with pytest.raises(ValueError, match="least 1 instance"):
+        pool = LocalMapdlPool(
+            0,
+            exec_file=EXEC_FILE,
+            nproc=NPROC,
+            additional_switches=QUICK_LAUNCH_SWITCHES,
+        )
+
+
+@requires("local")
+@skip_if_ignore_pool
+def test_only_one_instance():
+    pool = LocalMapdlPool(
+        1,
+        exec_file=EXEC_FILE,
+        nproc=NPROC,
+        additional_switches=QUICK_LAUNCH_SWITCHES,
+    )
+    pool_sz = len(pool)
+    _ = pool.map(lambda mapdl: mapdl.prep7())
+    assert len(pool) == pool_sz
+    pool.exit()