I have a few users using any connect VPN on ASA 55xx, now there is a requirement for MFA.
what are the pre requisites to use Duo security MFA?
Do I need a Radius server?
Any limitations in the Duo security solution?
Do I need any additional licenses in ASA?
See this post. The Duo Proxy Server can act as a RADIUS server, you don't necessarily need to deploy a separate RADIUS server to use Duo. You will obviously need a server (Windows/Linux) to run as the Duo Proxy.
The Duo link I provided, has very useful guides for all the different authentication scenarios (LDAPS, SML, RADIUS etc).
It depends on what features you require, this page https://duo.com/pricing has a break down on the features available for each license. It should be self explanatory.
You can contact Sales from that link or speak to your Cisco Account Manager.