Typically, we recommend installing Semplice on a live server environment (preferably on a temporary domain or folder on your server) instead of a local environment. This is because it will save you the hassle of having to transfer your local site to a live server environment later, which is not always an easy process.
However, you are absolutely free to install Wordpress/Semplice on a local server environment as a temporary place to build until you purchase a hosting plan.
For both Windows and OSX users, you can use MAMP to generate and install a local server environment on your computer and install Wordpress and Semplice. Here is a guide for installing Wordpress locally with MAMP.
You can install as many local instances of Semplice as you want using the same license key. This will not count as a license activation until your site is live on a domain.