, if you have to refer to a group of keys, then you should only say “ a bunch of keys”, because the collective noun ‘ bunch’, meaning same as the word ‘group’, matches only with the plural noun ‘keys’. So it is better to say ‘ A bunch of keys’ than ‘ A group of keys’. Likewise it is correct to say as follows,
A herd of cattle.
A fleet of ships.
A brood of hens etc…