OK, if a flame weapon strikes a horde, does the horde roll once for avoiding being hit by the weapon or does it roll once for every single hit put on it?

Hordes don't roll at all- they automatically take a number of hits based on the Flamer's range. The upside (from the Horde's point of view) is that they can't catch on fire.

Flamers are built for ruining a horde's day. Thus, like Adeptus said, they cannot dodge but simply get hit and take losses.

They are essentially kindling. I always keep a map on grid paper during every combat mostly for myself and I can judge the angle and spread of the flamer against the horde - this is useful if you have any elites tucked away in the horde that may have to roll to avoid damage should they get caught in the blast.

Always keep a map for yourself. I have found this invaluable, and my players really appreciate it as well.

Is that so? I've been having my hordes make the usual agility test to avoid being hit, I'm sure I googled that and found an official answer that hordes did indeed make the agility test. I'll have to recheck that then.

Like GMGM I was under the impression that Hordes did make a agility test as I found nothing in either the flamer rules or the horde rules that hordes did not get an agility roll.

Does anyone have a page number for the rule that the hordes does not get an agility roll?

No, hordes never catch on fire and never roll agility tests. Flame weapons affect hordes exactly as listed on page 360 of the rulebook, overiding the normal flamer rules.

This is reinforced via the errata which states:

Flame Weapons (page 360): Add the following sentence at the end of the description: “A Horde cannot be set on fire and does not suffer the lingering effects of fire (see page 260); instead any damage and effects of fire are assumed to be included in the Magnitude damage.”

Thanks. I've seen the part on 360 before but from the description of the rules it wasn't clear to me if hit means "takes damage" or hit means "roll to avoid getting struck by the flames." But since the general interpretation of the rules means automatic damage on the horde I shall rule it as such myself, as well.