You can choose any value for X as you cast Chamber Sentry. The value chosen for X doesn't directly affect the number of +1/+1 counters Chamber Sentry enters the battlefield with, but it does let you pay more mana and thus spend more colors of mana to cast it.
2018-10-05
wotc
The value of X chosen when you activate Chamber Sentry's first activated ability doesn't have to be the same value of X that you chose when you cast it.
2018-10-05
wotc
If Chamber Sentry enters the battlefield without being cast, it receives no +1/+1 counters.
2018-10-05
wotc
If another creature enters the battlefield as a copy of Chamber Sentry, consider what mana was spent to cast that creature to determine how many +1/+1 counters it enters with.
2018-10-05
wotc
If an effect allows you to spend mana as though it were mana of any color to cast Chamber Sentry, you consider only the actual mana spent to determine how many counters it receives.
2018-10-05
wotc
Colorless mana won't give Chamber Sentry another +1/+1 counter. Colorless is not a color.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Legal
Timeless
Legal
Gladiator
Legal
Pioneer
Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Not Legal
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