You can use the extension Regex Text Generator

Use the command Generate text based on Regular Expression

• for match regex use: .*
• for generator expression use: {{=i+1}}

As of v1.66 it won't be necessary to use an extension at all for simple replacement (0, 1, 2, ... or 1, 2, 3, ...) like this. New snippet variables

$CURSOR_INDEX or ${CURSOR_INDEX}
$CURSOR_NUMBER or ${CURSOR_NUMBER}

are being added (they are in Insiders now). So your snippet could be as simple as (inserted here via a keybinding in keybindings.json):

{
  "key": "alt+i",                       // whatever keybinding you want 
  "command": "editor.action.insertSnippet",
  "args": {
    "snippet": "$CURSOR_NUMBER"
  },
  "when": "editorTextFocus && editorHasMultipleSelections"   // if you want to restrict it
},

You can't do any real math on the added numbers other than increment from 0 or 1. To do more interesting math see below extension.

Using an extension that I wrote this is very easy: Find and Transform.

Create this keybinding (in your keybindings.json):

{
  "key": "alt+y",                   // whatever keybinding you want
  "command": "findInCurrentFile",
  "args": {
    // "find": "1",            // actually not necessary
    "replace": "${matchNumber}",
    "matchCase": true              // match only APPLE, not Apple
  }
}

That will replace whatever word your cursor is on with the matchNumber starting at 1. If you wanted to start at 0, use ${matchIndex}.

If you need to specify a more complicated regex find, you can do that too.