Hellkite Igniter‘s activated ability gives Hellkite Igniter +x/+0, where X is equal to the number of artifacts you control. Spells and abilities that ‘look’ at the game to get information do so when the spell or ability resolves. In this situation Hellkite Igniter’s ability needs to know how many artifacts are controlled by the controller of the Hellkite Igniter. That being the case, the game will not calculate the increase in power to the Hellkite Igniter until this ability resolves.

For example, lets assume that a player controlled three artifacts and activated the ability of his Hellkite Igniter. If another player were to respond to this activation by casting Shatter targeting one of these artifacts, then the Hellkite Igniter would only get +2/+0 and not +3/+0.