No matter how many are added you will still have to assume that people who are after a site and actually type in a domain name will pick the .com, so no matter how many alternatives you own you won't get that traffic.
The exception of course is when you can build it into your website name like .io and .ly. Also browsers are moving away from domain names so being first on Google (just as hard if someone prominent is already on the .com you want) will be more important.
If I'm after a UK or Australian company I'm as likely to try .co.uk or .com.au first - but only because I've lived in both of those places so I'm familiar with the extensions.
The exception of course is when you can build it into your website name like .io and .ly. Also browsers are moving away from domain names so being first on Google (just as hard if someone prominent is already on the .com you want) will be more important.