Hello,
Adaxes custom attributes are stored on Adaxes backend, which is ADAM or AD LDS, and thus any limitations to such attributes are also imposed by ADAM/AD LDS. There is no hard limit as to how many values you can store in a multi-valued attribute, because the limit is applied not to the number of values, but rather to the size of the attribute (in byte). The practical limit is around 1200-1300 values.
As an alternative to storing the groups in a custom attribute, you can use a muti-valued DN syntax attribute in your Active Directory, for example, See Also. If you store DNs of the groups a user was a member of in a DN syntax attribute, you'll be able to click on a group to view it's properties. For example, in the Web Interface, groups stored in See Also will be displayed like this: