How to create an array of ints when size of array and size of ints are not constant?

First, don't confuse an integer value with the size of the type used to represent it. The values 0 and 99 will take up exactly the same amount of space in whatever integral type you use.

As for the array itself, you have several options available.

You can pick a meximum number of elements that your user won't be allowed to exceed:

#define MAX_ALLOWED 100

int main( void )
{
  int values[MAX_ALLOWED];  // declare array with max-allowed values
  size_t howMany;

  // get howMany from user, make sure it doesn't exceed MAX_ALLOWED
  ...
}

If you are using a C99 or C2011 compiler that supports variable-length arrays, you could get the array length from the user and use that value in the array declaration:

int main( void )
{
  size_t howMany;

  // get howMany from user

  int values[howMany];  // declare array with user-specified number of values
  ...
}

If you don't want to use VLAs (and there are some good reasons why you don't), you can use dynamic memory management routines:

#include <stdlib.h>

int main( void )
{
  size_t howMany;

  // get howMany from user

  int *values = malloc( howMany * sizeof *values );  // dynamically allocate
                                                     // space to store user-
                                                     // specified number of
                                                     // values.
  if ( !values )
  {
    // memory allocation failed, panic
    exit( 0 );
  }
  ...
  free( values ); // release the memory when you're done.
}

Note that in the last case, values is not declared as an array of int, but a pointer to int; it will store the address of the memory that was dynamically allocated. You can still use the [] operator to index into that space like a regular array, but in many other respects it will not be treated the same as an array.

Regardless of how you create the array, assigning elements is the same:

for ( size_t i = 0; i < howMany; i++ )
{
  values[i] = value_for_this_element();
}