Learn French: Il y a beaucoup de fleurs au printemps. - There are a lot of flowers in spring.

In French, expressions of quantity like beaucoup (a lot), un peu (a little), and trop (too much/many) are always followed by just de (or d' before a vowel sound), never des. So it is always beaucoup de fleurs.

Why do we say au printemps (in spring) instead of using en like we do for other seasons?

Printemps is the only season that takes au (which is a combination of à + le). The other three seasons start with a vowel or silent h (été, automne, hiver) and use en without an article: en été (in summer), en automne (in autumn), and en hiver (in winter).

Why does the sentence use Il y a (there is) when we are talking about plural flowers (there are)?

Unlike English, which switches between 'there is' and 'there are', French keeps it simple. Il y a is used for both singular and plural things. You use it to say 'there is a flower' (il y a une fleur) and 'there are flowers' (il y a des fleurs).