The Red Heifer

"Here is another legal requirement commanded by the LORD: Tell the people of Israel to bring you a red heifer, a perfect animal that has no defects and has never been yoked to a plow. (Numbers 19:2 NLT)

Accord to "For Real, How Rare Is a Red Heifer? - Questions & Answers", these are the requirements of the red heifer:
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1. The cow must be, at a minimum, within its third year of life (i.e., two years plus a bit).
2. It needs to be completely red. Even two hairs of a different color next to each other or three that are far apart disqualify it.
3. All physical blemishes that disqualify sacrificial animals, disqualify a red heifer as well.
4. Any work done with it disqualifies the cow. "Work" in this case includes even a person leaning on it or placing a garment or cloth upon it (unless this was done to only safeguard the animal itself).
5. Placing a yoke on the cow, even if it doesn’t actually do any work, also disqualifies it.
6. If the heifer is pregnant, or even if a male has mated with it, it is disqualified.

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Quoting from the same source,
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There is, however, a common misconception about the rarity of the red heifer. Maimonides writes:
"Nine red heifers were offered from the time that they were commanded to fulfill this mitzvah until the time when the Temple was destroyed a second time. The first was brought by Moses, our teacher. The second was brought by Ezra. Seven others were offered until the destruction of the Second Temple. And the tenth will be brought by the King Moshiach; may he speedily be revealed. Amen, so may it be God's will."
Some take these words to mean that only nine qualifying red heifers have ever existed, and the birth of the tenth one will be a sign of the redemption. In truth, however, the ashes of a red heifer can last for a very long time, since only a very small amount was needed to be mixed with the purifying waters. So the fact that only nine were ever used for purification purposes does not mean that only nine were ever born, just that only nine were needed or used thus far.
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