You can find the list of customized strings for any language pack in the Language customization tool. For the filters, choose all the components, then check the box for "Customised only" and click the "Show strings" button. This will list them all.
If you have a lot of strings and several languages, and would like to get the totals, you can do this at the command line.
Customized strings are stored in /moodledata/lang/xx_local, where xx is the language code. So, lang/en_local or lang/es_local, etc. There will be a php file for each of the components you have customized which has one line that starts with $string for each string you have customized. Counting these lines in each php file will give you the total customizations for each language.
If you are on Linux, you can use grep to do this. For example, to see how many customizations I have made to the core English (en) pack, I usually use this command:
$ grep -ro '^\$string\[' /moodledata/lang/en_local/* | wc -l
This shows the total number. If you leave off the final part ( | wc -l ), it will show you the actual lines instead. You can repeat this for all the languages you have.