You don't NEED to add any URL channels.
As long as you have your Adsense ID inserted on this page: https://hubpages.com/my/earnings/affiliate/
It should look something like this: ca-pub-9407596374925154
Your Adsense adverts will be displayed for 60% of impressions on your hubs.
Channels are only needed if you want to track specific sites or hubs.
You can't have too many because the adverts will only ever show once for each impression. So say you have these TWO channels:
[Your subdomain or hubpages.com] - 300 views
[Hub Number 1] - 50 views
[Hub Number 2] - 250 views
It wouldn't matter - in the end you still only get 300 impressions/views. The channels are for tracking ONLY.