In its initial conception, Mr. Martin supposed it would be a trilogy, but part way through A Game of Thrones he decided it’d have to be four books. While working on A Clash of Kings, he came to realize that the story would require at least six novels, and for a long time he stuck to this. However, with the announcement of A Feast for Crows being completed in May 2005 (in part by splitting off some POVs which appeared instead in the fifth novel, A Dance with Dragons), GRRM started to state that seven books was the new target for the series. As recently as October 2014, he’s reaffirmed his intent to finish the series in seven novels.
According to GRRM’s official website, official titles for the books are: A Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, A Dance with Dragons, The Winds of Winter (forthcoming) and A Dream of Spring (forthcoming). A Time for Wolves is a possible title that GRRM has stated is in his contract, but it was one that he wasn’t happy with at the time.