A quick visit might be all right for that number of people - as long as they realise that it may well be too much for her and they would need to be prepared to leave quickly if that were the case - and not to be upset or take offence.
From my own experience, people with dementia beyond the early stages often don't realise it's Christmas anyway. By the time my mother was in her care home we would just visit her in the morning, taking her presents - which she was usually not interested in. Even before she went into her care home she wouldn't realise it was Christmas - even with crackers in the table etc. - and would wonder why people were giving her presents.
Beyond a certain quite early stage she couldn't cope with a lot of extra people, noise or fuss, and this was the case well before she went into the care home.
Were you planning just a visit, or to spend much of the day with her, or to,play it by ear?
I think it can be very hard for people without direct experience to realise the difference dementia can make. If I were you I think I'd tell them that she can no longer cope with extra people, noise and fuss (if this is the case).
And if you're not sure how long you'll be spending with her, tell them that, and that they would have to be prepared to 'do' Christmas (bring all the food and cook it!) without you, but you'd join them as and when.
However, if you'd really prefer them not to come at all, best to say that, though I know it might be very difficult if they're likely to be upset. You could say that in the very difficult circs at the moment you're just not up to having guests.
It may be different if they're the types who will see to everything and leave you free to visit for as long as you want, but if they're the sort of guests who are just going to give you extra work and stress, that's a different matter.
I do hope you find a solution that gives you the least stress.
