I never really get what they mean by accompanied bags & unaccompanied bags... If I am there filling in the form surely I am accompanying the bags- so I always put 0
Then I start thinking do they mean bags in the cabin vs bags in the hold... just seems silly.
& I always put 0 for family members travelling with me lol.
I recall decades ago (i was 15) going into UK from Tunisia, my aunt was with us & had 1 extra bottle of booze- my dad had filled my bag with a few bottles of booze ( I didn't know) & I had already gone through.
She got stopped & told them she was travelling with my mum & dad- so they got stopped - I was sat ages in the car park waiting.
Dad was furious at her for grassing up that she was travelling with them but they all forgot to mention me

So I guess the "number of family members travelling with you" bit is just to facilitate grabbing others in the group if you happen to have a
miscalculation on quantities.