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.
2015-08-25
wotc
Unless a spell or ability allows you to, you can't choose to pay more mana for a spell with a converge ability just to spend more colors of mana. Likewise, if a spell or ability reduces the amount of mana it costs you to cast a spell with converge, you can't ignore that cost reduction in order to spend more colors of mana.
2015-08-25
wotc
The maximum number of colors of mana you can spend to cast a spell is five. Colorless is not a color. Note that the cost of a spell with converge may limit how many colors of mana you can spend.
2015-08-25
wotc
If you cast a spell with converge without spending any mana to cast it (perhaps because an effect allowed you to cast it without paying its mana cost), then the number of colors spent to cast it will be zero.
2015-08-25
wotc
If there are any alternative or additional costs to cast a spell with a converge ability, the mana spent to pay those costs will count. For example, if an effect makes sorcery spells cost {1} more to cast, you could pay {W}{U}{B}{R} to cast Radiant Flames and deal 4 damage to each creature.
2015-08-25
wotc
If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.
Battle for Zendikar
Pioneer Masters