Whether (or “how many”) Hareidim serve/served in the IDF depends on definitions.
Many people define Hareidim in part as “not serving in the army.” By this definition, any Hareidi who serves in the army ceases to be hareidim (what the secular fanatics are hoping for). By this definition no hareidi has, is, or ever will serve in the army.
Once you broaden the definition to include such aspects as strict in kashrus and mitsvos (but that applies to all orthodox Jews), dressing “funny” (but that leaves out many such as the Frankfort au Main community), or holding that the Israeli state as now constituted does not have the power to pass laws that contradict halacha (but many Religious Zionists held the evacuation of settlments was against halacha, and considered armed resistance – does that make them Hareidi?) — you get a much larger hareidi community, many of who have always served in the army.