The documentation states:'... you may need to convert the UID string to allow for the multi-value field. If enough room exists for the multi-value field, add the field to the UID string... If there is not enough room within the current UID string for one value of the multi-value field, convert the UID structure to accommodate it. A multi-value field must be defined in the UID string as a full field; you cannot use a partial multi-value field as part of the UID string.'.
A multi-value field in the UID only includes the length of one field of the possible number of values in the multi-value string, not the length of the field times the number of possible values in the multi-value field.
The UID string can not exceed its maximum of 24 characters. There is NO extra overhead involved in the UID string length when defining a multi-valued field. If you define a new 7-byte multi-value field with 10 instances, the UID only needs to have the 7 bytes available not the full 70+ bytes that the multi-valued field uses in the logonid record. If you are re-defining an existing 7-byte field as a multi-value field there would be no change in the size or layout of the existing UID string.
Details on multi-value fields can be found in CA-ACF2 Security for z/OS Administrator Guide, Chapter 2: Controlling System Entry, section "Specifying Multi-Value logonid Fields and UID Strings", sub-section "Convert the UID String".