IFS=$'n';for file in $layers; do echo $file; done
cropped_1.png
cropped_2-a.png
cropped_2-b.png
cropped_2.png
cropped_3-a.png
cropped_3-b.png
cropped_3.png
cropped_4-a.png
cropped_4-b.png
cropped_4-c.png
cropped_4-d.png
cropped_4-e.png
cropped_4-f.png
cropped_4-g.png
cropped_4.png
cropped_5-a.png
cropped_5-b.png
cropped_5.png
--More--

i=1;IFS=$'n';for file in $layers; do echo ${file/#*_*/cropped_$((i++)).png}; done
cropped_1.png
cropped_2.png
cropped_3.png
cropped_4.png
cropped_5.png
cropped_6.png
cropped_7.png
cropped_8.png
cropped_9.png
cropped_10.png
cropped_11.png
cropped_12.png
cropped_13.png
cropped_14.png
cropped_15.png
cropped_16.png
cropped_17.png
--More--

i=1;IFS=$'n';for file in $layers; do mv $file ${file/#*_*/cropped_$((i++)).png}; done

ls cropped* | sort -nk1.9
cropped_1.png
cropped_141.png
cropped_142.png
cropped_143.png
cropped_144.png
cropped_145.png
cropped_146.png
cropped_147.png
cropped_148.png
cropped_149.png
cropped_150.png
cropped_151.png
cropped_152.png
cropped_153.png
cropped_154.png
cropped_155.png

i=1;IFS=$'n';for file in $layers; do mv $file ${file/#*_*/cropped_$((i++)).png}; done
mv: ‘cropped_1.png’ and ‘cropped_1.png’ are the same file
mv: ‘cropped_2.png’ and ‘cropped_2.png’ are the same file
mv: ‘cropped_3.png’ and ‘cropped_3.png’ are the same file
mv: ‘cropped_4.png’ and ‘cropped_4.png’ are the same file
mv: ‘cropped_5.png’ and ‘cropped_5.png’ are the same file
--More--

i=1;IFS=$'n';for file in $layers; do mv -v $file "${file/#*_*/cropped_$((i++)).png}"; done
‘cropped_2-a.png’ -> ‘cropped_2.png’
‘cropped_2-b.png’ -> ‘cropped_3.png’
‘cropped_2.png’ -> ‘cropped_4.png’
‘cropped_3-a.png’ -> ‘cropped_5.png’
‘cropped_3-b.png’ -> ‘cropped_6.png’
‘cropped_3.png’ -> ‘cropped_7.png’
‘cropped_4.png’ -> ‘cropped_8.png’
‘cropped_5.png’ -> ‘cropped_9.png’
‘cropped_6.png’ -> ‘cropped_10.png’
--More--

testy="cropped_2-b.png"
mv -iv $testy ${testy/#*_*/cropped_999.png}

i=998
testy="cropped_2-b.png"
mv -iv $testy ${testy/#*_*/cropped_$((i++)).png}

testy="cropped_3-a.png"
mv -iv $testy ${testy/#*_*/cropped_$((i++)).png}