How many comparables you actually need
Three to five tight ones. More is not stronger, and it is often weaker.
The instinct is that more evidence is more persuasive. In assessment appeals the opposite is usually true.
Why more can be worse
Every comparable you add is one the assessor can attack. If they find the weakest of twelve and show it is not comparable, the board's impression of your whole set drops — and you handed them the opportunity.
The right number
Three to five, each of which you can defend in one sentence. If you have found eight excellent ones, use the five closest and keep the rest in reserve for questions.
What "enough" depends on
Tightness, not count. Four parcels matching within 5% on every published attribute is a strong case. Fifteen matching loosely is a weak one, because the median of a loose set is not evidence about your property.
When you genuinely cannot find three
That is information. An unusual property may have no true comparables, in which case uniformity is not your ground and a market-value case — probably with an appraisal — is the route.