Whether or not it is true, the following has a clear sense:
1. The Hatfields outnumber the McCoys.
(1) says that the number of Hatfields is strictly greater than the number of McCoys. It obviously does not say, of each Hatfield, that he outnumbers some McCoy. If Gomer is a Hatfield and Goober a McCoy, it is nonsense to say of Gomer that he outnumbers Goober. The Hatfields 'collectively' outnumber the McCoys.
It therefore seems that there must be something in addition to the individual Hatfields (Gomer, Jethro, Jed, et al.) and something in addition to the individual McCoys (Goober, Phineas, Prudence, et al.) that serve as logical subjects of number predicates. In
2. The Hatfields are 100 strong
it cannot be any individual Hatfield that is 100 strong. This suggests that there must be some one single entity, distinct but not wholly distinct from the individual Hatfields, and having them as members, that is the logical subject or bearer of the predicate '100 strong.'
So here is a challenge to William the nominalist. Provide analyses of (1) and (2) that make it unnecessary to posit a collective entity (whether set, mereological sum, or whatever) in addition to individual Hatfields and McCoys.
Nominalists and realists alike agree that one must not "multiply entities beyond necessity." Entia non sunt multiplicanda praeter necessitatem! The question, of course, hinges on what's necessary for explanatory purposes. So the challenge for William the nominalist is to provide analyses of (1) and (2) that capture the sense of the analysanda and obviate the felt need to posit entities in addition to concrete particulars.
Now if such analyses could be provided, it would not follow that there are no 'collective entities.' But a reason for positing them would have been removed.