Declare @Sql nvarchar(max)
Declare @cnt varchar(10)
Declare @TableName as varchar(50)
SET @TableName = 'temp'

SET @Sql = 'Select @Cnt = count(*) From ' + @TableName
--PRINT @Sql
EXEC sp_executeSql @Sql, N'@Cnt varchar(10) output', @Cnt output
Select @Cnt