test.c

#include <assert.h>
#include <stdio.h>
#include <string.h>

size_t strchcount(const char *str,int ch)
{
	size_t count = 0;
	const char *s = str-1;
	while(s=strchr(s+1,ch))
	{
		count++;
	}
	return count;
}

int main(int argc,char **argv)
{
	assert(argc == 3);

	printf("%zu\n",strchcount(argv[1],*argv[2]));

	return 0;
}