I would use:
awk 'FNR <= 1' file_*.txt
As @Kusalananda points out there are many ways to capture the first line in command line but using the head -n 1 may not be the best option when using wildcards since it will print additional info. Changing 'FNR == i' to 'FNR <= i' allows to obtain the first i lines.
For example, if you have n files named file_1.txt, ... file_n.txt:
awk 'FNR <= 1' file_*.txt
hello
...
bye
But with head wildcards print the name of the file:
head -1 file_*.txt
==> file_1.csv <==
hello
...
==> file_n.csv <==
bye