spark plug #5

This is very standard for most V6 and V8 engines out there.

Looking at the FRONT of the engine (in the Honda Odyssey, this means standing on the Right side looking at the serpentine belt area):

Now using your own body anatomy as reference point....

Your (L) hand area is the first bank of the engine and is numbered from front to back.
If V6, then it is 1-2-3
If V8, then it is 1-2-3-4

Your (R) hand area is the second bank of the engine and is numbered from front to back.
If V6, then it is 4-5-6
If V8, then it is 5-6-7-8

See picture...