DECLARE @sessionsCount int = 1;
DECLARE @timeLimitBetweenSessions int = 5;

SELECT
  --ttt.id, rownum23, trt, ttt.CreateDateTime, DATEDIFF(minute, pl2.CreateDateTime, pl1.CreateDateTime)
		a.Id, a.User_Id,
		b.User_Id,
		a.CreateDateTime StartActivity,
		b.CreateDateTime EndActivity,
		a.RowNum1,
		b.RowNum2,
		@sessionsCount as 'SessionNumber'
  FROM (
  select Id,
		User_Id ,
		CreateDateTime
		,ROW_NUMBER() OVER (ORDER BY [User_Id]) AS RowNum1
  from  [dbo].[PageLoadings]) as a
  JOIN (
  SELECT [Id]
      ,[User_Id]
      ,[CreateDateTime]
	  ,ROW_NUMBER() OVER (ORDER BY [User_Id]) AS RowNum2
	    FROM [dbo].[PageLoadings]) as b

		on a.RowNum1 = (b.RowNum2 - 1)