Below are a couple of examples to quick start.
{exp:has_entries channel="channel_name_1|channel_name_2" parse="inward"}
<ul>
{has_entries:channel_data}
<li>channel_id: {has_entries:channel_data:channel_id}</li>
<li>site_id: {has_entries:channel_data:site_id}</li>
<li>channel_name: {has_entries:channel_data:channel_name}</li>
<li>channel_title: {has_entries:channel_data:channel_title}</li>
<li>channel_url: {has_entries:channel_data:channel_url}</li>
<li>total_entries: {has_entries:channel_data:total_entries}</li>
<li>total_comments: {has_entries:channel_data:total_comments}</li>
<li>last_entry_date: {has_entries:channel_data:last_entry_date}</li>
<li>last_comment_date: {has_entries:channel_data:last_comment_date}</li>
{/has_entries:channel_data}
<li>entry_count: {has_entries:entry_count}</li>
</ul>
{exp:channel:entries channel="channel_name_1|channel_name_2"}
...
{/exp:channel:entries}
{/exp:has_entries}