In addition to strtok which was already mentioned, your platform may provide a regcomp function for regular expressions:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <regex.h>

int main(void)
 {
   char *header = "This is a sample header\nNumber: 12345\nThis is ano
+ther line\n";
   char *number = NULL;
   regex_t re;
   regmatch_t pmatch[2];
   int retval;

   regcomp(&re, "^Number: (.+)", REG_EXTENDED | REG_NEWLINE);
   retval = regexec(&re, header, sizeof(pmatch)/sizeof(*pmatch), pmatc
+h, 0);
   if(retval == 0 && pmatch[1].rm_so != -1)
    {
      size_t len = pmatch[1].rm_eo - pmatch[1].rm_so;

      number = malloc(len + 1);
      memcpy(number, header + pmatch[1].rm_so, len);
      number[len] = '\0';
      printf("Number is %s\n", number);
    }
   else
    {
      printf("Match failed\n");
    }

   regfree(&re);
   free(number);
   return 0;
 }
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No error checking is done and number is stored as a string, atoi()/strtol() family can turn it into an integer.