Apparently the limit is actually 20.

davidben · davidben · commit 985a000379fb · 2013-11-03T01:35:42.000-05:00
diff --git a/manage/index.html b/manage/index.html
@@ -71,4 +71,4 @@ <h3>Are you sure?</h3>
   </fieldset>
 </form>
 <div id="create-alert-placeholder"></div>
-<p><em>Limit of 10 databases.</em></p>
+<p><em>Limit of 20 databases.</em></p>
