You can choose any value for X. Choosing a higher value for X lets you pay more mana and thus spend more colors of mana to cast it. For example, if you choose 0 for the value of X and pay {U}{U} to cast Sweep the Skies, you'll create one Thopter. If you choose 4 for the value of X and pay {W}{U}{U}{B}{R}{G}, you'll create five Thopters.