You can choose any value for X. The value chosen for X doesn't directly affect the number of +1/+1 counters Skyrider Elf enters the battlefield with, but it does let you pay more mana and thus spend more colors of mana to cast it. For example, you can choose 0 for the value of X and pay {G}{U} to cast Skyrider Elf for two +1/+1 counters. You can also choose 3 for the value of X, making its mana cost {3}{G}{U}, and pay {W}{U}{B}{R}{G} for five +1/+1 counters.