What does 500多个 mean?
N 多 个 = over N:
Pleco has a bunch of sample sentences:
If you type 多个 open the entry then click SENTS you’ll get something like this:

Which features sentences like:

一百多个人
yībǎi duō ge rén
more than 100 people

and

他们有保留曲目二十多个。
They have a repertoire of over 20 songs.

So 500多个 can be 501 - 599, basically.