Learn Dutch: Watjes heb ik er niet veel nodig. - Cotton pads, I don't need many of them.

This is an example of split topicalization. By putting just the bare noun (watjes) in the first position, you emphasize it as the topic ('As for cotton pads...'). The quantifier (niet veel) stays behind in the middle field of the sentence.

Why is the word er necessary in this sentence?

Because watjes is a countable noun that has been separated from its quantifier (veel). In Dutch, when you leave a number or quantity of a countable noun dangling without the noun right next to it, you must use the quantitative er. It translates roughly to 'of them' in English.

The lesson example Geld heb ik niet veel does not use er. Why does this sentence need it?

It depends on whether the fronted noun is countable or uncountable. Geld (money) is uncountable, so it does not trigger the quantitative er. Watjes (cotton pads) are countable, so when you split the phrase, er is absolutely required to point back to the fronted noun.

Why is nodig placed at the very end of the sentence?

The phrase used here is nodig hebben (to need). In Dutch main clauses, the conjugated verb (heb) takes the second position, while the other part of the expression (nodig) gets pushed to the end of the sentence, much like a separable prefix.