scountf(char*) - counts the number of replacement strings in a format string, sorta

scountf.c

static int
scountf(const char * format)
{
    int count = 0;
    int index = 0;
    char this, next;
    
    while ((this = format[index])) {
        next = format[index++];
        
        if (this == '%' && next != '%') { // skip %%
            count++;
        }
        else if (next == '*') { // surrender
            return -1;
        }
    }
    
    return count;
}

/* Alf Watt - alf@istumbler.net - assigned to the public domain */

80% of the time it works 100% of the time

If you're using the GNU C library, you can checkout my answer on SO: https://stackoverflow.com/a/66624722/11293716
Or the manual page directly for a built-in solution: https://www.gnu.org/software/libc/manual/html_node/Parsing-a-Template-String.html
And please don't use this gist in production, if you don't want to surrender (;