To answer your first question: If you have clients that do not support TLS 1.2, and these clients connect to your server using TLS 1.0 or TLS 1.1, and you disable TLS 1.0 and TLS 1.1 on your server, then these clients will not be able to connect to your server.
With regard to the pro's and con's of continuing to support TLS 1.0 and TLS 1.1: Many organizations have disabled (or will soon disable) TLS 1.0 and TLS 1.1, because of known vulnerabilities in these versions of TLS. See this article for more information on these vulnerabilities. Disabling TLS 1.0 and TLS 1.1 on your server will protect your server and your clients from these vulnerabilities. However, if you have clients that support TLS 1.0 and/or TLS 1.1, but not TLS 1.2, then these clients will not be able to connect to your server if you disable TLS 1.0 and TLS 1.1.
Having said that, all mainstream web browsers have supported TLS 1.2 for quite some time now, so disabling TLS 1.0 and TLS 1.1 will probably affect very few of your users, if most of your users connect to your site with a recent version of a mainstream web browser.
For what it's worth, StackExchange recently disabled TLS 1.0 and TLS 1.1. for all of their sites, and posted a very informative write-up of the process here. Interestingly, they found that less than 1% of the TLS connections to their servers used TLS 1.0 and TLS 1.1 (and they suspect that most of these connections were from bots).
With regard to your final question about determining how many of your clients are still connecting to your server using TLS 1.0 and TLS 1.1, most web servers have the ability to log the TLS version that was used for each request from a client. Check the documentation for your web server to see how to enable this.
Related: What will happen to older browsers if I disable SSLv3 on my webserver?