Print number of variables in repr (#4762)

* Print number of variables in repr

* Tweak title only when there's many rows

Workaround to avoid having to redo every single doctest... It is really only necessary when the data rows are limited. But I find it a bit difficult to count the rows quickly past like 7.

* Remove min() 

No need to limit max_rows now because the if condition handles that.

xarray/core/formatting.py

@@ -372,7 +372,9 @@ def _mapping_repr(mapping, title, summarizer, col_width=None, max_rows=None):
         max_rows = OPTIONS["display_max_rows"]
     summary = [f"{title}:"]
     if mapping:
-        if len(mapping) > max_rows:
+        len_mapping = len(mapping)
+        if len_mapping > max_rows:
+            summary = [f"{summary[0]} ({max_rows}/{len_mapping})"]
             first_rows = max_rows // 2 + max_rows % 2
             items = list(mapping.items())
             summary += [summarizer(k, v, col_width) for k, v in items[:first_rows]]