Filtering or repeating elements as needed
I think if you look at the objects within the IEnumerable as a statistical numeric value than you could use the term Normalize. In this case you are "normalizing" your values (objects) to a desired count.

public static IEnumerable&lt;T&gt; NormalizeObjects&lt;T&gt;(this T[] items, int desiredCount)


If you dislike the term Objects you could replace it with Items or even Records depending on what your use case is. 

If the only thing you dislike about your current name is "Trim" then you could replace that term with the opposite of Expand, which I would argue is Contract.

public static IEnumerable&lt;T&gt; ExpandOrContract&lt;T&gt;(this T[] items, int desiredCount)


I think either of the above are decent options based on what your method is currently doing.