The option may not be present for an Admin to uncheck the "One per Record" area on an Attribute Category from a Attribute.
There is no way for this to be simply unchecked. When a Attribute Categories is being added there is an option to check "Allow only one per record", once that is chosen and saved the option can not be unchecked by default. The workaround at this point is to manually delete the Attribute and re add it without checking that box. there is a suggestion in Idea bank for this to change https://bbcrm.ideas.aha.io/ideas/CRM-I-2388