How can I use sprites as numbers?

Yep, you can do this. It's a little work though. The basic idea is that you get each digit of the number which will be from 0 through 9, and use that as an index into an array of prefabs for the equivalent digit image...

 // initialize a sprites array with the a prefab for for each digit,
 // making sure the 0 index points to the 0 sprite image, and the 9 index
 // points to the 9 sprite image - left as an exercise for the reader - 
 // should be able to do this in the editor if you make it a public property

 GameObject[] sprites = new GameObject[10]
 
 int value = 4143;  // make sure the data type is an integer
 
 // this is the key part, converting an integer score into your sprites
 while (value > 0)
 {
   // get the 1's place value - % is the modulo operator which
   // divides by the the specified value and returns the remainder, 
   // e.g. 4143 % 10 = 414 with a remainder of 3, so it returns the value 3
   // which is the digit in the 1's place
   int digit = value % 10;
 
   // TODO : create and position sprite instance from sprites[digit]
   // remember that we're going from the right to left
 
   // move the 10's place to the 1's place
   // the first time through value is 4143, so dividing by 10 gives us 414
   value /= 10;
 }

Note that you're building things from right to left - i.e. each time through the loop you're getting the value in the ones place. You'll have to have a special case where value starts out at 0. And maybe some other edge cases I didn't think of. But this should give you the basic idea.