Comparing a couple different function implementations.

paper_word_count.c

#define _POSIX_C_SOURCE 200809L

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

#define PENCIL_SPACE ' '

size_t paper_word_count(char *paper)
{
    size_t count = 0;
    char *ptr = paper;

    do {
        if (!*ptr) {
            break;
        }

        if (*(ptr - 1) == '\0' && *ptr != PENCIL_SPACE) {
            count += 1;
        }

        if (*(ptr - 1) == PENCIL_SPACE && *ptr != PENCIL_SPACE) {
            count += 1;
        }
    } while (*(++ptr));

    return count;

    /*
        size_t count = 0;
        char *ptr = paper;

        for (size_t pos = 0, len = strnlen(paper, BUFSIZ); pos < len; ++pos, ++ptr) {
            if (pos == 0 && *ptr != PENCIL_SPACE) {
                count += 1;
            }

            if (pos >= 1 && *ptr != PENCIL_SPACE && *(ptr - 1) == PENCIL_SPACE) {
                count += 1;
            }
        }

        return count;
    */

    /*
        size_t count = 0;
        const char *delimiter = " ";
        char *state = NULL;
        char *word;

        word = strtok_r(paper, delimiter, &state);

        if (word == NULL) {
            return count;
        }

        count += 1;

        while((word = strtok_r(NULL, delimiter, &state))) {
            count += 1;
        }

        return count;
    */
}