More than 64 parameters to WHERE IN sentence makes MSSQL reading far too many rows
An IN clause gets "rewritten" to this form, essentially:

WHERE 
    TableID = 260905384
    OR TableID = 260915601
    OR TableID = 260929877
    ...


I have heard, and observed, that SQL Server has a hard coded limit of 64 OR predicates that it will put in a scan or seek operator.  This isn't publicly documented anywhere as far as I can tell, and there isn't any way I'm aware of to change it either.

Beyond 64 OR expressions, as you mentioned, you end up with the "Constant Scan" plan, which drives multiple seeks or scans into the index (one for each literal).

Putting that many values literal values in an IN construction is generally considered a bad idea.  If possible, change the way the query is written.  You could, for example, insert all of those values into a temp table, and then do an INNER JOIN to that temp table table on the TableID column.