A good portable awk shebang is not easy to find.

shebang.awk

#!/usr/bin/awk -f
#!/usr/bin/awk -E
#
# If you have an awk version that doesn't support the -f flag,
# then you are just out of luck.
#
# If you just have no clue where awk will be, or you prefer to use -E,
# then you can try this bash snippet to launch awk for you.
#
# You might think the -E tests below are overly complex. You'd be wrong.
# I thank Apple for their wonderfully broken awk on Mac OS X.
# If you find a system, this doesn't work on please let me know!
#
# Copy the below line to the top of the file:
#!/usr/bin/env bash
#
# Inspired by: https://unix.stackexchange.com/a/97280
#
_src="${0}"
function exec_awk() {
  _cmd="$(command -v awk)"
  _awk_prog="/awk:/ && /option/ && /-E/ {exit 1;} {next}"
  _flag="-$("$_cmd" -E /dev/null </dev/null 2>&1 | "$_cmd" "$_awk_prog" && echo 'E' || echo 'f')"
  true bash#; exec "${_cmd:-false}" "${_flag}" "${_src}" "$@"; exit;
}

true {}#; set -e; exec_awk "$@"

# You place your awk program below this line and make sure this file has
# execute permissions.

do you actually need either -f or -E ?

dash -c '________='\''BEGIN { for(_ in ENVIRON) { print _, length(ENVIRON[_]) } }'\''; 

printf "\n\n\tawk \47%s\47\n\n" "$________"; mawk2 -- "$________" | gcat -n ' 


	awk 'BEGIN { for(_ in ENVIRON) { print _, length(ENVIRON[_]) } }'

     1	m2aCMB 37
     2	mnetHnOdFn 32
     ….
    12	HOMEBREW_PREFIX 13
    13	NODE_REPL_HISTORY 30
    14	mmvre1 43
    15	NODE_REPL_HISTORY_SIZE 5
    16	SHLVL 1
    17	TMPDIR 49
    18	m3rOLD 44
    …….
   121	ZDOTDIR 16
   122	m3t 41

worked exactly the same for gawk ::::

   124		# gawk profile, created Thu Jan 12 10:53:31 2023
   125	
   126		# BEGIN rule(s)
   127	
   128		BEGIN {
   129	   123  	for (_ in ENVIRON) {
   130	   123  		print _, length(ENVIRON[_])
   131			}
   132		}
   133	

in fact, if your shell allows this, directly piping in commands via /dev/stdin also works :::

         dash -c ' __='\''BEGIN { for (_ in FUNCTAB) { print _, __ = FUNCTAB[_], length(_) } }'\''; 
                          
                         printf "%s" "$__" | gawk -p- -- "$( paste -)" | gcat -n ' 

     1	rand rand 4
     2	dcgettext dcgettext 9
     3	gsub gsub 4
     4	match match 5
     5	int int 3
     6	log log 3
     7	sprintf sprintf 7
     8	strftime strftime 8
     9	systime systime 7
    ...
    37	sub sub 3
    38	substr substr 6
    39	xor xor 3
    40	lshift lshift 6
    41	strtonum strtonum 8
    42	toupper toupper 7
    43		# gawk profile, created Thu Jan 12 11:15:06 2023
    44	
    45		# BEGIN rule(s)
    46	
    47		BEGIN {
    48	    42  	for (_ in FUNCTAB) {
    49	    42  		print _, __ = FUNCTAB[_], length(_)
    50			}
    51		}
    52	

but of course, if it supports -f ( which is just about every major awk variant actually being used) - you can even automate multi-awk code testing without needing eval or exec :

 dash -c '__='\''BEGIN { srand(); srand(); OFS = "\f"; 

                          print ENVIRON["_"], ARGV[+_], srand(), NR, FPAT, OFMT, CONVFMT, ARGC, srand() }'\''; 

            printf "\n\n\t \"\$awkvariant\" \47%s\47\n\n" "$__"; 

            for ___ in "nawk" "mawk" "mawk2" "gawk"; do 

                    printf "%s" "$__" | "$___" -f- | gcat -n; 
                    printf " %s\n\n"      "$___"

           done ' 


	 "$awkvariant" 'BEGIN { srand(); srand(); OFS = "\f"; print ENVIRON["_"], ARGV[+_], srand(), NR, FPAT, OFMT, CONVFMT, ARGC, srand() }'

     1	/bin/dash
                 nawk
                     1673540748
                               0

                                %.6g
                                    %.6g
                                        1
                                         1673540748
 nawk

     1	/bin/dash
                 mawk
                     1673540748
                               0

                                %.6g
                                    %.6g
                                        1
                                         1673540748
 mawk

     1	/bin/dash
                 mawk2
                      #srand1673540748.39820#
                                             0

                                              %.6g
                                                  %.6g
                                                      1
                                                       #srand1673540748.39824#
 mawk2

     1	/bin/dash
                 gawk
                     1673540748
                               0
                                [^[:space:]]+
                                             %.6g
                                                 %.6g
                                                     1
                                                      1673540748
 gawk

The goal is to have awk run the content of the file without introducing additional dependencies. (Using bash is a fallback that's not preferred.)

As the comments say, if you already know where awk will be and/or which variant of awk will be used, and you don't care about supporting any unknown systems, then just write the shebang you need using that knowledge.