The plural of client is clients.The collective noun of clients is clientele.
Clientele is singular: a 'grouping' term like 'flock' or 'horde' that lumps multiple individual units into a single term. So, "During its heyday, the shop's clientele was representative of the finest in high society."
noun
The noun oxygen is a singular, uncountable noun, a word with no plural form.
The noun 'onlooker' is a single compound word, the plural is onlookers.
The noun 'onlooker' is a single compound word, the plural is onlookers.
The noun 'fungi' is the plural form of the noun 'fungus'.
The noun audience is a singular, common noun. The noun audience is also a collective noun.
A single invitation is different from plural invitations.
"Pounds" is typically seen as a plural noun, used to refer to a unit of weight.
There is no plural form for mankind. The noun mankind is an uncountable (mass) noun, a word for all humans considered as a single group, a word for a concept.
The plural form of the noun ''country'' is countries.