The AnyConnect client software is free for all devices - mobile on iOS and Android, or computer with Windows, Mac OS X, or Linux. As noted in the data sheet, an active SMARTnet contract (whether or not it is directly purchased or PSRT type) which includes at least one ASA is required to be able to access and download the client software images for distribution to computers (either directly via software installer packages or via an ASA using the pkg files). The mobile versions are free and can be downloaded on the respective marketplace - Apple App Store for iOS or Google Play for Android.
To enable remote access VPN on an ASA (beyond the included two free AnyConnect Premium licenses), you need to buy licenses. There are various types available including AnyConnect Essentials (one license per a given appliance or HA pair which allows basic remote access up to the capacity of the device), Premium (sold in license packs - i.e 50 clients, 100 clients etc. and gives enahnced features for the licensed number of clients counted as simultaneous users), and AnyConnect for Mobile (requires one of the first two and necessary for any mobile clients (iOS or Android).
There are some other miscellaneous license types but that covers the majority of use cases.
