Believe it or not, the title was going to be Six Samurai, but after Kurosawa decided that they needed someone to shake things up and added Kikuchiyo to keep everybody on their toes, it turned into seven. It's a pretty self-explanatory title, describing the seven heroes who are going to save the day. Its simplicity matches the mood he wants to convey.
It also provides a handy little hook that gets audiences' attention. Lucky seven holds a lot of significance to Western audiences, helping this movie hook the international crowd, but it's an even more important number to the Japanese.
Japanese myths talk about the Seven Gods of Fortune, and Buddhists claim that a baby is given his soul on the seventh day. In fact, Buddhism puts a huge emphasis on seven as the number of life and death, as well as part of the cycle that defines the moon and the movements of the stars. (The cool thing is that all of that developed independently of the "lucky seven" stuff in the West.)
He might not have planned for it, but the number gives Kurosawa's title a nice little kick that helps people take notice of it a little more. Let's face it: it just wouldn't sound as cool if it were Six Samurai, and it might not have gotten the attention it did without plugging into some of those cultural connotations.
Guess Kikuchiyo really did save the day, didn't he? More evidence that Toshiro Mifune makes everything better.