Try a global command:
:g/^/exe ".w! line".line('.').".txt"
:g/^/
Do a command for every line (you can adjust this regular expression if you only want to save certain lines, i.e..
for non-empty lines)exe ""
execute the following command.w!
save the current line and overwrite if already exists. (Remove!
if you don't want to auto-overwrite everything)"line".line('.').".txt"
dynamically insert the line number.
See :h :g
:h :g
and :h execute
:h execute
for more info.