Self-hosted web fonts are priced by monthly pageviews — a pageview being defined by every time a visitor to your site loads each page. Start by reviewing whatever traffic analysis tools you use and choose the plan that covers that number. A website which receives 1.5MM pageviews/month, for example, should choose the 2MM plan.
You can upgrade your plan from your Font Library at any time for the remainder of your term at a pro-rated cost, and if your average website traffic increases beyond your current plan for three months in a row, the self-hosted web fonts license requires you to upgrade to the next tier. Permanent upgrades are also available for perpetual plans.