I have been searching through the technical specification of Okta and have been unable to find the max amount - I have found the following:
https://help.okta.com/en/prod/Content/Topics/Directory/Reference_Directories.htm
This talks about quite a bit but really doesn't answer the login attribute question - it does give quite a bit of information about email, which if we follow the RFC's and the addendums to the RFC's that number is 254 characters. Found here: https://www.rfc-editor.org/errata/eid1690
There, however, is additional information that seems to indicate that attributes carry no limit - which can't be the case, especially in regards to email addresses:
It has changed over the years. Currently (and just tested this in an okta.com tenant), the maximum total length the Okta login and primary email fields is 100 characters total combined (prefix and suffix).