The speaker presents us with a list of natural phenomena, all of which are characterized by multiplicity. So, for example, he starts off by telling us that there are hundreds of stars in the pretty sky. He then goes on to refer to the hundreds of shells on the shore together. After that, we're reminded that hundreds of birds go singing by and that there are a similar number of bees in the sunny weather.
At the beginning of the second and final stanza of this very short poem, the speaker says that there are "Hundreds of dewdrops to greet the dawn." By this, he simply means that when dawn breaks the grass, it is covered by hundreds of little dewdrops. What the speaker is doing here is to contrast the multiplicity of nature with the uniqueness and the singularity of his mother.
There may be countless dewdrops, stars, seashells, birds, and bees in the world, but there's only one mother. Even the presence of hundreds of lambs in the purple clover and hundreds of butterflies on the lawn can't change this basic fact. The speaker's mother, like the speaker himself, is part of this natural world yet is at the same set apart from it by her uniqueness as a human being.