I agree with DVK that 7 is the most magical number in Potterverse because canon (i.e. Tom Riddle) says so.
According to Miller's Law - The Magical Number Seven, Plus or Minus Two, seven is the approximate amount of single items a person is able to retain in their working memory at any one time. It could be that seven was a number of hiding locations that Voldemort could most easily remember, thus seven Horcruxes.
I ran ‘Tom Marvolo Riddle' through The Sorcerer's Companion Arithmancy Calculator and got these numbers:
Character Number: 7
Heart Number : 6¹
Social Number: 1²
Perhaps Tom Riddle found value in his Arithmancy chart. Remember he immediately believed Dumbledore when he was eleven and Dumbledore told him he was a wizard; he outright stated that he had known he was special all along. If seven - the most magical number - came up on his Arithmancy chart, he may have been eager to see this as a reinforcement of his belief that he was the most magical being in the world, and subsequently he would protect his life with seven Horcruxes.
A 7 means: Perceptive, understanding, and bright, sevens enjoy hard work and challenges. They are often serious, scholarly, and interested in all things mysterious. Originality and imagination are more important than money and material possessions. Sevens can also be pessimistic, sarcastic, and insecure.
There are seven years of magical schooling through Hogwarts, when witches and wizards really come into their own, increase their powers, learn to control magic, and prepare to go out into the world and do great things ("Terrible! But great. . .")
¹ I'm not even going to pretend that Voldemort has a heart.
² This fits Voldemort as well. A 1 means: This is the number of the individual, the solitary unit. Ones are independent, focused, and determined. They set a goal and stick to it. They are leaders and inventors. Ones find it difficult to work with others and don't like to take orders. They can be self-centered, egotistical, and domineering. They are often loners.