pack of 6 filters
six-pack of filters
etc.

I wouldn't really use "six-pack", though I can't explain why.

Support "pack of 6 filters".

Perhaps you associate it too much with beer and abs?

I, like no me bré, have no problem with "six-pack of filters".

In dem Zusammenhang fällt mir eine andere Frage ein:
Kann man auch z.B. "five-pack" sagen, wenn das, worum es geht, nur aus fünf Teilen besteht? Oder eben jede x-beliebeige Zahl? Habe bislang nämlich nur six-pack gehört.

No, Robert, that's not why.

ACB asks a compelling question. I'd also opt for "pack of five XXX".

While I certainly have no issue with "pack of five XXXs", I also have no issue with "five-pack" or "any other number"-pack. Five-pack may sound strange only because so few things are packed that way. Six-pack, ten-pack and twelve-pack are probably the most common.

Ffive and I obviously disagree on this. That happens. Perhaps others will add their opinions.

Edit: #3 should have had a smiley after the first sentence. :-)

I don't know why, but I would say for a six-pack, twelve-pack, etc, all individual items must be visible and arranged in a symmetrical manner, maybe that's Ffive's point? :-)