From what I have read Google allows 3 unit per page. I tested it, and only managed to show 3.

Channels are a way of grouping your ad units. This allows you to track the data for each channel individually.

Sometimes Google may display a PSA instead of an actual paid ad. PSAs don't generate any income, so you have the option of filling in the ad space with a solid colour or displaying an ad from another site. PSAs usually show up if Google can't find any related ads.

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