#define PP_COMMASEQ_N()                                    \
          1,  1,  1,  1,                                   \
          1,  1,  1,  1,  1,  1,  1,  1,  1,  1,           \
          1,  1,  1,  1,  1,  1,  1,  1,  1,  1,           \
          1,  1,  1,  1,  1,  1,  1,  1,  1,  1,           \
          1,  1,  1,  1,  1,  1,  1,  1,  1,  1,           \
          1,  1,  1,  1,  1,  1,  1,  1,  1,  1,           \
          1,  1,  1,  1,  1,  1,  1,  1,  0,  0

#define PP_COMMA()    ,

#define PP_HASCOMMA(...)                                   \
          PP_NARG_(__VA_ARGS__, PP_COMMASEQ_N())

#define PP_NARG(...)                                       \
          PP_NARG_HELPER1(                                 \
              PP_HASCOMMA(__VA_ARGS__),                    \
              PP_HASCOMMA(PP_COMMA __VA_ARGS__ ()),        \
              PP_NARG_(__VA_ARGS__, PP_RSEQ_N()))

#define PP_NARG_HELPER1(a, b, N)    PP_NARG_HELPER2(a, b, N)
#define PP_NARG_HELPER2(a, b, N)    PP_NARG_HELPER3_ ## a ## b(N)
#define PP_NARG_HELPER3_01(N)    0
#define PP_NARG_HELPER3_00(N)    1
#define PP_NARG_HELPER3_11(N)    N

PP_NARG(1)

-----
$ gcc -E main.c

# 1 "main.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 321 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "main.c" 2
# 27 "main.c"
PP_NARG_HELPER3_PP_NARG_(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0main.c:27:1: error: pasting formed ')PP_NARG_', an invalid preprocessing token
PP_NARG(1)
^
main.c:16:11: note: expanded from macro 'PP_NARG'
          PP_NARG_HELPER1(                                 \
          ^
main.c:21:37: note: expanded from macro 'PP_NARG_HELPER1'
#define PP_NARG_HELPER1(a, b, N)    PP_NARG_HELPER2(a, b, N)
                                    ^
main.c:22:59: note: expanded from macro 'PP_NARG_HELPER2'
#define PP_NARG_HELPER2(a, b, N)    PP_NARG_HELPER3_ ## a ## b(N)
                                                          ^
) PP_NARG_(PP_COMMA 1 (), 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0)(PP_NARG_(1, PP_RSEQ_N()))
1 error generated.