The purpose is to assassinate Bellatrix, then? Did Montague confirm that she never attends the festivals? All my intel has indicated that she rarely leaves the Palace, so that would align with what I know.

There will still be some guards and staff on hand at the Palace. I'm sure she knows not to leave the place completely vulnerable. Broome will be there, for certain. Lana S-P wouldn't dream of missing a party, I bet, so she'll be gone.

Actually, let me just send you a list of every MLE person I saw out and about at the last festival. That should give a rough idea of who will be left behind, and how many.