Segmentation fault with concatenation of two strings

answer to new edits to OP

Edits to OP include initialization of variables addressing some instances of undefined behavior in original code, however buffer overflow problem remains in following section:

/* Generator data */
  for(i = 0; i < number_cpu; i++)
  { 
    for(j = 0; j < 3; j++){
      name_cpu[j] = array_letters[rand() % (sizeof(array_letters)-1)];
      array_numbers_final[j] = array_numbers[rand() % (sizeof(array_numbers)-1)];
  }
   strcat(name_cpu, array_numbers_final);
   fprintf(list_cpu, "%s \n", name_cpu);
  }
  fclose(list_cpu);

Where name_cpu is defined with 8 char: char name_cpu[8] = {0}

"If number_cpu is equal to 1 it works. But if it is higher then 1, the program goes into segmentation fault."...

After first iteration of outer loop, including one strcat of name_cpu and list_cpu, the char name_cpu is populated with 6 new characters and \0 in remaining locations. Eg:

//example
|S|J|P|Y|E|P|\0|\0|  //

The error occurs at end of 2nd iteration of outer loop at the strcat() statement. The inner loop re-populated the first three positions of name_cpu, eg:

//first three positions changed, the rest remains
|C|B|A|Y|E|P|\0|\0|  // 
 ^ ^ ^ updated values

Error occurs when the new value for array_numbers_final :

|G|H|O|\0|\0|\0|\0|\0|

is attempted to be concatenated to name_cpu, resulting in:

|C|B|A|Y|E|P|G|H|O|  //attempt to write to memory not owned
                ^end of buffer

Resulting in buffer overflow error, and likely the segmentation fault error condition. Again, as before, design variables to meet the potential needs of of the program. In this instance the following will work:

 char name_cpu[100] = {0};
 char array_numbers_final[100] = {0};     

answer to original post:
The problem is here:

strcat(name_cpu, array_numbers_final);//undefined behavior.

First, name_cpu[8] is uninitialized at time of declaration, nor is it initialized at any time before used. Because it is not guaranteed what the contents are, it is not necessarily a string. Using this variable as is in any string function can invoke undefined behavior

Furthermore,(even if name_cpu is a valid string) when array_numbers_final is concatenated to name_cpu, the buffer will overflow. During my test run I saw this:

The fix is to start off with initialized buffers of sufficient size for intended purpose. eg:

 char name_cpu[100] = {0}
 array_numbers_final[100] = {0},    
 array_letters[27] = "ABCDEFGHIJKLMNOPQRSTUWXYVZ",
 array_numbers[10] = "123456789";

Code example below is adaptation using your provided example, with suggested edits. Read in-line comments:

int main(void)
{
    int number_cpu = 3;//added - previously not defined in OP
    int i = 0,          
    j = 0;     
    char name_cpu[100] = {0},//resized and initialize
     array_numbers_final[100] = {0},//resized and initialize    
     array_letters[27] = "ABCDEFGHIJKLMNOPQRSTUWXYVZ",
     array_numbers[10] = "123456789";


/* Generator data */
      for(i = 0; i < number_cpu; i++)
      { 
          for(j = 0; j < 3; j++){
          name_cpu[j] = array_letters[rand() % (sizeof(array_letters)-1)];
          array_numbers_final[j] = array_numbers[rand() % (sizeof(array_numbers)-1)];
        }

        strcat(name_cpu, array_numbers_final);
        fprintf(stdout, "%s \n", name_cpu);//to stdout
        //        ^ changed here
      }
    return 0;
}