How many hard drives does P5LD2 Deluxe support?

Solution
The Crucial scan report says nothing about how many internal drives your motherboard supports.

Your motherboard supports 5 internal SATA drives.
Plus 3 IDE (PATA) drives.

These are divided between three separate controllers (Intel, ITE, & Silicon Image) as shown on the specs sheet:
https://www.asus.com/Motherboards/P5LD2_Deluxe/specifications/

However, that doesn't necessarily mean you can actually install that many as it depends on how many internal drive bays the PC case has..
The Crucial scan report says nothing about how many internal drives your motherboard supports.

Your motherboard supports 5 internal SATA drives.
Plus 3 IDE (PATA) drives.

These are divided between three separate controllers (Intel, ITE, & Silicon Image) as shown on the specs sheet:
https://www.asus.com/Motherboards/P5LD2_Deluxe/specifications/

However, that doesn't necessarily mean you can actually install that many as it depends on how many internal drive bays the PC case has..
 
Solution
Crucial Scan cannot detect empty drive bays because there are no connected devices in them, and it has no clue what your case model is.

Your case, unless it's an old factory-built PC, is almost certain to have more than one internal drive bay.
And no, affixing a drive with sticky tape is a big no-no. Heat will soften the tape adhesive and the drive will fall, which will break it.

Open your case and take a look - drive bays should easily be visible. Or post case model.