If you have the Advanced Security licence, you just need to buy Cisco AnyConnect licences.
You need one per person who will be using AnyConnect. Usually, you buy it with a term to match your Meraki licence (for example, 3 or 5 years).
When buying AnyConnect there are two main options - AnyConnect Plus and AnyConnect Apex.
With APEX you can do SAML authentication - which means you can directly authenticate against things like Azure AD, which makes doing MFA really easy.
If SAML is of no interest, get AnyConnect Plus instead (cheaper).