Just solved it. Used another int.

random_number:
mov ah, 2Ch    
int 21h        

mov al, dl     
xor ah, ah     
mov cx, 10     
div cl         

mov dh, ah     
ret

Now when returning to generate_enemies:

generate_enemies:
mov ah, 0x13
mov al, 0x01
mov bh, 0
mov bl, 0x07
mov cx, 1
mov bp, enemy_pixel

mov dl, 79
int 0x10
ret

DH is already setted up with AH that matches its size.