The original 5150 PC had 5 slots. The followup XT had 8 (one nonstandard) because 5 was skimpy, especially before high integration-- if you needed a whole slot

The original 5150 PC had 5 slots. The followup XT had 8 (one nonstandard) because 5 was skimpy, especially before high integration-- if you needed a whole slot for just a serial port or a battery backed clock, and seperate cards for floppy and hard discs, they ran short fast.