DD2 was just like that when she learned to count. She was about 17mo, she woke me up in the middle of the night and counted to 5 for me. A few weeks later, she woke me up again to count to 11. Now she knows all the numbers to 20, though usually she gets to 11 and starts back at 7 or 8.
She found a set of number flashcards I'd bought for DS4 from the $1 bin at Target. He wanted nothing to do with them, but she is absolutely enthralled. She brings them to me and says "Mommy, talk to me about the numbers" and then proceeds to sit for 20 minutes while we go through the numbers, counting the dots on each card, looking at the words for each number and trying to name each numeral.
I'd say that if you can find a cheap set of cards, see if she likes them. If she does, fantastic, play on! If not, then just put them away for later. I was really worried about hot housing for awhile. But, I've come to the conclusion that as long as they enjoy it and you completely follow their lead on when to stop, then there's no harm in exposing them to new things and playing new games.
