The speaker starts by imagining a big, showy act: painting a thousand suns. They quickly turn it down, because a crowd of painted suns would feel hollow. The only way it would mean anything is if the suns were meant for everyone, and the speaker then realizes that this is already true of the one real sun. It belongs to everybody, so one is plenty.
The final two lines shift from argument to picture. The sun is simply there in the sky, shining "like new pennies." That comparison is small and a little playful. Pennies are ordinary, everyday things, but new ones are bright and shiny, so the image makes the sun feel both humble and precious.
The mood is quiet, gentle and content. The speaker doesn't sound frustrated or lonely, just settled, as if they've decided that making more isn't necessary. The poem suggests that something shared and already present can be worth more than a grand creation, and that noticing it is enough.