--- a/hgext3rd/topic/__init__.py	Sun Aug 14 20:40:46 2016 +0200
+++ b/hgext3rd/topic/__init__.py	Fri Aug 12 23:59:37 2016 +0200
@@ -289,6 +289,13 @@
             fm.plain(' %s ' % marker, label=label)
         fm.write('topic', '%s', topic, label=label)
         fm.data(active=active)
+        if ui.verbose:
+            # XXX we should include the data even when not verbose
+            data = stack.stackdata(repo, topic)
+            fm.plain('\t(')
+            fm.write('changesetcount', '%d changesets', data['changesetcount'],
+                     label='topic.list.changesetcount')
+            fm.plain(')')
         fm.plain('\n')
     fm.end()
 

--- a/hgext3rd/topic/stack.py	Sun Aug 14 20:40:46 2016 +0200
+++ b/hgext3rd/topic/stack.py	Fri Aug 12 23:59:37 2016 +0200
@@ -64,6 +64,16 @@
         fm.plain('\n')
         fm.end()
 
+def stackdata(repo, topic):
+    """get various data about a stack
+
+    :changesetcount: number of non-obsolete changesets in the stack
+    """
+    data = {}
+    revs = repo.revs("topic(%s) - obsolete()", topic)
+    data['changesetcount'] = len(revs)
+    return data
+
 # Copied from evolve 081605c2e9b6
 
 def _orderrevs(repo, revs):

--- a/tests/test-topic-list.t	Sun Aug 14 20:40:46 2016 +0200
+++ b/tests/test-topic-list.t	Fri Aug 12 23:59:37 2016 +0200
@@ -173,10 +173,10 @@
 verbose
 
   $ hg topic --verbose
-     bar
-     baz
-   * foo
-     fuz
+     bar	(5 changesets)
+     baz	(2 changesets)
+   * foo	(2 changesets)
+     fuz	(3 changesets)
 
 json
 
@@ -206,18 +206,22 @@
   [
    {
     "active": false,
+    "changesetcount": 5,
     "topic": "bar"
    },
    {
     "active": false,
+    "changesetcount": 2,
     "topic": "baz"
    },
    {
     "active": true,
+    "changesetcount": 2,
     "topic": "foo"
    },
    {
     "active": false,
+    "changesetcount": 3,
     "topic": "fuz"
    }
   ]