Purchasing a plugin for a multi-site network

Hello~

I have a multi-site network set up with my site. It has 6 different sections that function as the overall site (see https://www.ultimatehealthtrends.com ).

I am interested in purchasing the pro version of the plugin, Social Slider. Would I have to purchase it for each individual sub-site, or would purchasing it with the main site be sufficient?

Thanks in advance for any advice on this!