In the Genesis Ark story, one pair of unclean animals enters the Ark. The animal instructions distinguish between unclean animals and clean animals brought in greater numbers.
Genesis 6:19–20 gives a general instruction to bring two of every kind, male and female. Genesis 7 then adds a more specific instruction: seven pairs of each kind of clean animal and one pair of each kind of unclean animal. The later passage is why readers often see two different numbers in summaries of the account.
The clean-animal provision is associated with sacrifice and with preserving suitable animals after the Flood. Genesis does not provide a complete scientific classification of which species counted as clean or unclean in this passage, so applying modern zoological categories would go beyond the text.
A common mistake is to say that every animal entered in seven pairs. That number applies to clean animals in the Genesis account; unclean animals are represented by a single male-female pair.