Why does the Torah write here that two of each species came to Noach, when of the Tahor species there were seven pairs?
Rashi: The Pasuk means that at least two of each species came. 1
Ramban: Two (a male and a female) of each and every species came by themselves (as the Pasuk specifically states), and Noach went and fetched the six remaining pairs of Tahor animals that were meant for Korbanos. 2
One could perhaps answer that of every species two came, irrespective of the fact that of the Tahor species there were more.
QUESTIONS ON RASHI
Rashi writes: "'They came to Noach' - of their own accord." How is this derived from the verse?
Gur Aryeh: Otherwise, the verse should have stated, "... they were brought to Noach."
Rashi writes: "'They came... of their own accord." Earlier, however, Noach was commanded to bring two of each (6:19)?
Gur Aryeh #1: The command was to bring them on to the Teivah, once they arrived.
Gur Aryeh #2: Noach was commanded to "bring them in" verbally, by saying, "Come in to the Teivah."
Rashi writes: "'Two by two' - they were all similar regarding this number." What does this mean?
Gur Aryeh: Rashi was troubled that the Kosher species numbered seven and seven, not merely two. Rather, the verse means that each species had at the very least two.
Rashi writes: "They were all similar regarding this number;" i.e. each species had at least two. However, this is not the usual way in which the Torah uses numbers?
Gur Aryeh: Alternatively (unlike Rashi), one may explain that all of the animals came in pairs, i.e. even the Kosher species came as seven pairs and not as individuals.