To assign numbers in ascending order when replacing the same number

For Vim 7.4.765 or later, g<C-a> after selecting the target in visual mode generates a serial number.

Just in case, I'll write down other methods.

Number == line number is acceptable:

:%s/^\d\+/\=line(".")

If you want to start over from 1 in the selected range in visual mode:
(Shift+v and press : to automatically enter :'<,'>)

: '<, '>s/^\d\+/\=line(".")-line("'<")+1

It's a little complicated, but it's a macro version.

<CTRL-A>:%s/^0//
ggqqyw+P<CTRL-A>0q2@q

• <CTRL-A>
  • Add to 0 on the first line
  • 10<CTRL-A>
  if you want to jump 10
• :%s/^0//
  • Clear the rest of the line 0
• ggqyw+P<CTRL-A>0q
  • Return to the beginning of the statement (gg) Start recording to register q (qq)
  • Yank words (yw) Go to the beginning of the next line (+) Paste to the left of the cursor (P)
  • Add (<C-A>) to the cursor position number (pasted previous line number)
  • Back to line beginning (0)
  • End Recording (q)
• 2@q
  • Specify the number of iterations and run the macro in register q with @q

• Add to 0 on the first line
• 10<CTRL-A>
if you want to jump 10

• Clear the rest of the line 0

• Return to the beginning of the statement (gg) Start recording to register q (qq)
• Yank words (yw) Go to the beginning of the next line (+) Paste to the left of the cursor (P)
• Add (<C-A>) to the cursor position number (pasted previous line number)
• Back to line beginning (0)
• End Recording (q)

• Specify the number of iterations and run the macro in register q with @q