You can use dynamic sql

DECLARE @sql nvarchar(4000) 
...
--SET @RowCount = (SELECT COUNT (*) FROM @TableName)
SET @sql = 'SELECT @RowCount = COUNT (*) FROM ' + @TableName
EXEC sp_executesql @sql, N'@RowCount int OUTPUT', @RowCount OUTPUT

>I am trying to create a DB design document for a legacy DB. Part of this is the row count of each table. I have put together the following query, but it chokes at the line indicated (inside the cursor).
>
> Msg 1087, Level 15, State 2, Line 44
> Must declare the table variable "@TableName".
>
>All help is always appreciated.
>
>Line of error:
>

>  	SET @RowCount = (SELECT COUNT (*) FROM @TableName)
>

>
<snip>