Hello Peter - One 4’x8’ pillow would be sufficient for a 21’ pool. It should be positioned in the middle of the pool.
You’ll have a pretty big mess to clean up if you do not use a cover while the pool pump is not running. There is no point to using a pillow if you do not use a cover.
Yes, it is a “pillow” filled with air. It is used under winter covers on above-ground pools. If it is placed in the center of the pool, it allows rainwater to build up around the edge of the pool instead of in the middle of the cover. The other idea behind using a pillow is that it allows water to expand up instead of out (against the pool walls).