How are so few producers able to support so many primary consumers?

Because primary producers get 100% energy consumption straight from the sun.

Explanation:

We use the energy pyramid to help us out:

.01% - A pex Predators

0.1% - Tertiary Consumers

1% - Secondary Consumers

10% - Primary Consumers

100% - Primary Producers

Because Primary Producers get 100% of the energy of the sun, they can support so many primary consumers, who only get 10% of the energy when they consume primary producers.