The first-party WordCount plugin is already included in Craft’s CKEditor plugin. It can show word and/or character counts and include logic on what should happen as you type (e.g. when you get near or over the limit). If you toggle “Show word count” in the CKEditor field’s setting, you get the word count shown underneath the field. To turn on character count, go to CKEditor Config used by that field and add the following under “Config Options”:
"wordCount": {
"displayCharacters": true,
"displayWords": true
}