${en_number:u} would convert $en_number (the output from number) to upper case.  The :u modifier is zsh-specific and ${variable:u} corresponds to ${variable^^} in bash.
Testing:
$ for name in file_{1..10}.pdf; do touch "$name"; sleep 2; done
$ ls -lt
total 4
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_10.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_9.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_8.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_7.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_6.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_5.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_4.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_3.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_2.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_1.pdf
-rw-r--r--  1 kk  wheel  163 Jan 31 13:26 script.sh
$ zsh script.sh
$ ls -lt
total 4
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_10_TEN.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_9_NINE.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_8_EIGHT.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_7_SEVEN.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_6_SIX.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_5_FIVE.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_4_FOUR.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_3_THREE.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_2_TWO.pdf
-rw-r--r--  1 kk  wheel    0 Jan 31 13:28 file_1_ONE.pdf
-rw-r--r--  1 kk  wheel  163 Jan 31 13:26 script.sh
 
                