It's up to the game to display the counts on the screen when needed. SNK recommends using a 8x8 fix layer font.
The credit count for each player is stored in BCD in $D00034 for player 1, and $D00035 for player 2. These locations are in the backup RAM, to keep the credit counts in case of a power failure during play.
Note: SYSTEM_IO must be called each v-blank to update those locations.
moveq.l #0,d0 move.b $D00034,d0 ; Read P1 credits (hex) move.w #$7088,REG_VRAMADDR ; Set position in fix map, top left move.w d0,d1 ; Make a copy of the BCD value move.w #$20,REG_VRAMMOD ; Auto-inc fix column lsr.b #4,d1 ; Get tens andi.w #$000F,d1 ; Mask off eventual garbage ori.w #$5030,d1 ; Set palette 5 and tile offset $30 (ASCII "0") move.w d1,REG_VRAMRW ; Write tens to VRAM andi.w #$000F,d0 ; Get units ori.w #$5030,d0 ; Same move.w d0,REG_VRAMRW ; Write units to VRAM