It was over 3000 pagers that exploded. This article tries to explain it.

The evidence so far suggests that Hezbollah acquired the pagers around February when the group’s leader, Hassan Nasrallah, ordered members to stop using cellphones because they could be easily intercepted and monitored by Israeli intelligence.

In this case, Magnier said, the pagers procured by Hezbollah were with a third party and they sat at a port for three months, awaiting clearances, before they were finally moved to the Lebanese group.

Hezbollah suspects that it was during those three months that Israel managed to plant explosives in the devices, the military analyst said.

A Lebanese security source and another source told the Reuters news agency that the Mossad, Israel’s spy agency, planted explosives in 5,000 pagers that Hezbollah had ordered months before the explosions. The sources said a code was simultaneously sent to 3,000 of the pagers, triggering the explosions.

https://www.aljazeera.com/news/2024/9/18/how-did-hezbollah-get-the-pagers-that-exploded-in-lebanon