Each package comes with a number of Keyword Rank Credits per month.  A credit is consumed every time a keyword is rank checked in a 'Search Engine'.  You can configure as many search engines as you need for each project. e.g. Clients typically check their rankings on their local country version of Google for desktop and mobile users, which is two different 'Search Engines'.
A change in search engine and country (Google.co.uk, Google.fr, Baidu.com, Bing.co.uk, etc), device (mobile or desktop), language or query location (national or from a specific location) creates a new 'Search Engine' and will require a fresh rank check and consume an additional Keyword Rank Credit.
For example, to check 100 keywords daily using a desktop device in Google.co.uk from Liverpool would consume 3,000 Keyword Rank Credits a month (#Keywords * #Search Engines * Frequency = 100 * 1 * 30 = 3,000 Keyword Rank Credits).
To check the same hundred keywords daily using a desktop and mobile device would consume double the number of credits (#Keywords * #Search Engines * Frequency = 100 * 2 * 30 = 6,000 Keyword Rank Credits).