Well RD gave you a formula that would only work in the first column (A),
what column() returns in the first column is 1, then copied across it will
return 1,2,3 and so on. However when you put it in column E it will start
from 4, then 5,6 etc thus when you came to a certain column it bypassed the
dimension of the INDEX formula and thus returned the #NUM! error.
COLUMNS($A$1:A1) will return 1 wherever you put it and since I used absolute
reference for A ($ signs) it will increase the same way but it is more
robust than column(). So it has nothing to do with A it is just there to
make the formula increase the SMALL values 1st, 2nd, 3rd and so on which is
what "sorts" the values

If you enter:
=Column()
in say F1, you'll see that the return is 6.

This is the value of the column that the formula *resides* in.
When I suggested the formula, I aimed it (not wisely) to start in Column A.

As you drag the formula (=Column() ) across, you'll see that it *doesn't
change* in relation to it's location.

You could enter:
=Column(C1) in F1, and you'll get a return of 3.
As you drag that formula across, it will change (D1, E1, F1, ...etc.) to
increment the value as it changes resident columns.

So, you could just as well have used:

=SMALL($E53:$L53,COLUMN(A1))

And still returned the correct answers.

Don't forget, the actual formula is:
=SMALL($E53:$L53,1)
Where "1" denotes the *first* position (smallest value) in the array E53 to
L53.
We're using Column(), or Column(A1), or Columns($A$1:A1), to *automatically*
increment the position in the array in every formula so that you won't have
to physically change that value in every formula you enter along the row:
=SMALL($E53:$L53,1)
=SMALL($E53:$L53,2)
=SMALL($E53:$L53,3)
... etc.

The formula:
=Columns($A$1:A1)
is very robust, in that it always starts out returning a "1", no matter
where (which column) it is entered into,
and it will withstand the insertion and/or deletion of columns and still
return the correct value.

You could also use it to start out returning *any* number:
=Columns($A$1:C1) = 3
=Columns($D$1:H1) = 5

And you could also use it to *decrement* values:

=COLUMNS(A1:$J$1) to start out at 10, and then go *down* as you drag it
across.