"Groente" is an uncountable noun in Dutch. It is similar to the way "fruit" is used in English. If I say "I have fruit", that usually refers to multiple pieces of fruit. Likewise, "groente" refers to multiple vegetables.
No, it is possible to use the plural. But in that case it specifically refers to the types of vegetables. Just like in English you can say: "Apples and pears are different fruits".
