What size heat pump for 21 foot round pool?

The size of the heat pump is not the true question in this case. Most, if not all produce enough btu to heat a pool that size. The true question on a heatpump is how long. The larger the btu rating of the heatpump... the faster the pool water will reach desired temp.

Heatpumps should only be used to maintain heat in a pool. The most inefficient way to use a heatpump is to try and raise the pool temp a large temperature differintial at any given time. This type of "on demand" heat will create a scenario where the heatpump runs continuous at its highest capacity until desired temp is obtained, and the highest setting of "heat" on a heatpump is its most inefficient usage of energy. This type of energy usage is only an acceptable practice early in the spring when attempting to raise the pool temp for the first time that anual season.

The recommended use for a heatpump is to set the swim temp and allow the heat pump to maintain the temp to extend the swim-ability of the pool one to two months early in the spring and late in the fall. Maintaining the pool temp is a much more efficient use of energy than trying to use a heatpump to create "on demand" heat.