As the wiki states, you need a sprite for each slot. For the WAD to work at all, you will need at LEAST the following sprites (AAAA stands for whatever four-letter prefix you choose):
AAAAA0
AAAAB0
AAAAC0
AAAAD0
AAAAE0
AAAAF0
AAAAG0
AAAAH0
AAAAI0
AAAAJ0
AAAAK0
AAAAL0
AAAAM0
AAAAN0
AAAAO0
AAAAP0
AAAAQ0
AAAAR0
AAAAS0
AAAAT0
AAAAU0
AAAAV0
AAAAW0
AAAAX0
AAAAY0
AAAAZ0
AAAA[0
AAAA\0
AAAA]0
AAAA^0
AAAA_0
AAAA`0
AAAAa0
AAAAb0
Of course, the zeroes mean that the same sprite will be shown for all eight directions. You can split them to show different sprites for different directions. For example, AAAAB0 can become AAAAB1, AAAAB2B8, AAAAB3B7, AAAAB4B6, and AAAAB5. For details on that, see the wiki.
If you wish to save yourself the hassle of putting placeholder sprites in manually, you could instead edit a copy of an existing character WAD, in which case the placeholder sprites will be ready-made for you. I did exactly that when making Bippo, and that’s why all my sprites for that character are preceded by TAIL.
Additionally, without the c and d sprites, I believe you will be unable to gain 1UP boxes (the e sprite might possibly also be required, but I am unsure of that), which will be a problem. Without the f sprite, the signpost will be removed as soon as it spins to what would normally be your picture, but that is purely aesthetic.