1)
What is the meaning of "Pikdu"?
1.
Rashi: Take an accounting, like "va'Tifkedenu li'Vkarim" (Iyov 7:18).
2.
Radak: It is remembrance and supervision, like "Pakadti Eschem" (Shemos 3:16).
2)
Why did Sha'ul command about this?
1.
Malbim: He wanted to know what caused this commotion.