Learn Danish: Der er næsten ingen fiskere der arbejder her i dag. - There are almost no fishermen who are working here today.

Danish strongly prefers this presentational construction when an indefinite subject (like næsten ingen fiskere) is performing an action. While you could technically say Næsten ingen fiskere arbejder her i dag, it feels very heavy and unnatural to a Danish ear. Putting the indefinite subject after Der er and attaching the action in a relative clause with der is the standard way to phrase this.

Can I use som instead of the second der?

Yes, you can absolutely say ...fiskere som arbejder her i dag. When the relative pronoun acts as the subject of the relative clause, both der and som are grammatically correct. However, the double-der structure (Der er ... der ...) flows very well and is extremely common in everyday Danish.

Could I use ikke nogen instead of ingen in this sentence?

Yes, you could say Der er næsten ikke nogen fiskere... and it would mean the exact same thing. In everyday spoken Danish, ikke nogen is actually slightly more common, whereas ingen can sometimes sound a bit more formal or typical of written language. Both are perfectly correct here.

If I wanted to add an adverb and say 'who actually work here today', where would faktisk go?