This question is not a duplicate as the epithet śatakratu itself has a separate developmental history and the answer to the general question elsewhere is somewhat naive and incomplete by providing only the much later meaning attached to that epithet.
Indra's epithet as śatakratu occurs originally in the Rig Veda dozens of times, most of the times as an invocative śatakrato and did not mean "One who performed One Hundred Sacrifices" but "One with a Hundredfold insight or power".
Sāyaṇa (14th century) glosses śatakrato as "bahukarmayuktendra" = "Oh Indra, You who are skillful in manifold deeds".
Monier-Williams dictionary gives: "having hundred-fold insight or power or a hundred counsels etc."
Of the 46 occurrences of śatakrato, here are some:
RV 1.4.8: asyá pītvā́ śatakrato gʰanó vr̥trā́ṇām abʰavaḥ
RV VI.41.5: śátakrato mādáyasvā sutéṣu prā́smā́m̐ ava pŕ̥tanāsu prá vikṣú
It was only during the much later epic period that it was reinterpreted as meaning "One who performed One Hundred Sacrifices".