A lot of them were there from the very beginning. They showed several other girls among the core cast even in the first few episodes but they were always relegated to the background.
Cast In The Name Of God…Ye Not Guilty

Yeah, these girls were always there is the background, just weren't core characters. There were always more than 5 girls there, obviously. They introduce them as needed, Lost style. Next season we will meet The Others, the survivors from the tail section that crashed on the other side of the mountain.
The Lion does not give a fuck. Bring. More. Sheep.

I mean **** you can stab a tree these days and it gives you rubber! Who knows what they were packing back in '96!
My password is password.

Have you not watched how to make your own paper, magnetic? Even the egyptians could do that.
My password is password.

They have an entire airplane's interior and luggage to go through to make various things. Christ, if you are going to make an effort to harp on questionable details, ask yourself how Shauna managed to accumulate an armful of journals out there.

I just discovered CENSORED in Ben's Deer pit! She isn't pit girl (as of yet)! Good grief thank god there's another episode!
My password is password.

This **** sounds like a soap opera. Jfc
My password is password.

And where the **** did they get chinese lanterns in 1996 in the middle of the woods? I can't.
My password is password.

Stfu mate I didn’t notice these things!
Stop.

Ok, well I'd say you do have a point about the "feast"…it is implied that everyone other than Ben partook in them, so It is noteworthy that only the core cast is seen there…hard to explain that away from a good narrative perspective.
Cast In The Name Of God…Ye Not Guilty

I've not seen writing as bad as this since pretty little liars and the ptsd is still going strong
My password is password.

The depicted number of actual girls in the woods has been fluctuating from ten to eighteen since the show's first season, and they've hired different girls to play them over time. This is on purpose, and when questioned, the show-runners have openly said so.
So this has been going on since day one. Either accept that the people making the show actively don't give a crap about this sort of thing, or quit watching if you actually think this **** is important.

Where the **** were they when the cabin burnt down? During all the feasts/hunts/drama? They haven't been in a single ****ing scene in 4 years and I'm expected to believe they were there in the background all this time collecting water and emptying the **** bucket? Fuck off
My password is password.

Fucking communists.
My password is password.

I should have known you would watch this trash show.
My password is password.

There's nothing to explain–there are main characters and generic background characters.
Don't worry about the girls in the background, they are just noise. When you need to pay attention, the show will give the ones you need to watch some dialogue and actions. The others don't matter.

Oh so I'm supposed to read up about **** that isn't explained in the show to make the show make sense? Great stuff!
My password is password.

Keeping track of the characters and storyline is too complicated
I'm going to go back to watching reality TV
My password is password.

Relax. They are fudging the number of girls and the actresses playing them on purpose, and have openly said so. This gives them room to introduce new characters as needed.