To get certificates, you use client software, normally running on your web server, that submits the certificate requests, normally makes requested changes to prove your control over the sites that will be covered by the certificates, and then downloads (and in some cases automatically installs) the resulting certificates. The available client applications are listed at
The total number of sites that you host isn't super-huge by Let's Encrypt standards (some hosting providers have issued on the order of a million certificates for their customers!), but it might be a bit of a system administration or scripting challenge to figure out how you want to handle certificate installation and renewal, especially if you have new clubs coming in and old clubs leaving (or letting their domains expire) with some frequency.