Almost. I think you're reflexively doing the same thing GP was questioning (which I agree with; in the original example the new variable was just straight duplication of knowledge and is as likely to be the source of bugs as anything else (like if items were added or removed, it's now out of date)).

Here though you're missing the ".Count", so

  var relevantItems = items.Where(x => x.SomeValue > 5);
  relevantItems.Count

As long as it's not a property that's calculated every time it's accessed, this still seems better than pulling the count into its own variable to me.