The creature you put the counters on doesn’t have to be one of the tapped creatures.

Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain’s bolster ability.

You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.

Count the number of tapped creatures you control as the ability resolves to determine the value of X.