10 girls in the class.
OK to invite 6 (three not invited)?
OK to invite 7 (two not invited)?
To add to complications, DD came home today saying that S was mocking L because she hasn't been invited to R's party. "Isn't she like your best friend? So how come you aren't invited?" type of mocking.
DD then told L she can't invite her to her party (first I've heard of her having a party 
) as there won't be space, but she'll invite her over separately.
So, yes, DD has already invited 6 people to her party, including S. We have had words, along the lines of you don't invite people to parties without clearing it with me first... and I've said considering how nasty S was to L and how DD joined in, she should switch out S for L. I may be biased as S has previously stolen from our house before.