I've never used Qualtrics myself and do not need to, but my company receives Qualtrics-generated CSV data from another company, and we have to advise them about the names to use for fields/variables, such as "mobilephone".
The main thing I need to know is the maximum number of characters, but other limits (such as special characters to avoid) would be helpful. For example, would profile_field_twenty6chars be good? (The data is going into Moodle, which uses profile_field).
Qualtrics has 2 different limitations to my knowledge.