;

WITH part
AS (
	SELECT a.[regstamp]
		,a.[ID]
		,a.[f1]
		,a.[f2]
		,a.[f3]
		,a.[f4]
		,a.[f5]
		,ROW_NUMBER() OVER (
			ORDER BY 1 / 0
			) / 100000 + 1 [N]
	FROM [act] a
	)
	,N
AS (
	SELECT DISTINCT N
	FROM part
	)
SELECT N.N
	,t.j
FROM N
CROSS APPLY (
	SELECT a.[regstamp]
		,a.[ID]
		,a.[f1]
		,a.[f2]
		,a.[f3]
		,a.[f4]
		,a.[f5]
	FROM part a
	WHERE N.N = a.N
	FOR json path
	) t(j)

insert into #tmp(j)
Select * from [act] a
Order by ID
OFFSET @i*100000 ROWS 
   FETCH NEXT 100000 ROWS ONLY
for json path

if Object_id('tempdb..#t') is not null
Drop table #t

Create table #t(N int identity(1,1), j nvarchar(max))

declare @i int
Declare part cursor local FORWARD_ONLY READ_ONLY 
for
	SELECT DISTINCT N = number
	FROM master..[spt_values]
	WHERE number >= 0
	and number <= (Select count(*) / 100000 + 1 from [act])

OPEN part  
  
FETCH NEXT FROM part INTO @i 
  
WHILE @@FETCH_STATUS = 0  
BEGIN  

insert into #t(j)
Select * from
(
	SELECT a.[regstamp]
		,a.[ID]
		,a.[f1]
		,a.[f2]
		,a.[f3]
		,a.[f4]
		,a.[f5]
	FROM [act] a
ORDER by a.ID
OFFSET @i*100000 ROWS 
   FETCH NEXT 100000 ROWS ONLY
for json path
) t(j)

FETCH NEXT FROM part INTO @i   
END   
CLOSE part;  
DEALLOCATE part;