The X in (1,2,3) type of searches is only available for fields, and would be used like so:

{[]:[MyField](all) in (1,2,3)}

or

{[]:[MyField](any) in (1,2,3)}

The "all"/"any" only matters for multivalued fields: if there are multiple values in the field, "all" requires that all field values be present in the list and "any" requires that one or more of the values are present in the list.

Fred, the syntax you described {LF:ID = 1} | {LF:ID = 2} is the correct way to search for multiple entry IDs. However, depending on your needs, you might be able to use wildcards to achieve the search with a single term.

If you are searching for a range such as 12340 to 12359, you could use bracket wildcards:

{LF:ID="123[4-5][0-9]"}