declare @t table (value int, dt	datetime)
insert into @t values
 (778530593,	'2019-07-16 09:00:30.583')
,(778808056,	'2019-07-16 09:05:08.057')
,(779035029,	'2019-07-16 09:08:55.040')
,(779482297,	'2019-07-16 09:16:22.330')
,(779718789,	'2019-07-16 09:20:18.833')
,(779725674,	'2019-07-16 09:20:25.717')
,(787176545,	'2019-07-16 11:24:36.597')
,(787182633,	'2019-07-16 11:24:42.683')
,(787182633,	'2019-07-25 11:24:42.683')

-- ***
DECLARE @DateBegin datetime ;
DECLARE @DateEnd datetime ;

SET @DateBegin	='2019-07-15 22:00:00.000';
SET @DateEnd	='2019-07-25 22:00:00.000';

select 
	v.number
	,dt_start	=dateadd(hour, 12 * v.number, @DateBegin) 
	,dt_finish	=dateadd(hour, 12 * (v.number + 1), @DateBegin)
	,N			=count(a.value)
from spt_values v
outer apply (select * from @t t where t.dt >= dateadd(hour, 12 * v.number, @DateBegin) and t.dt < dateadd(hour, 12 * (v.number + 1), @DateBegin)) a
where v.type = 'P'
	and dateadd(hour, 12 * v.number, @DateBegin) < @DateEnd
group by 
	v.number

DECLARE @DateBegin datetime ;
DECLARE @DateEnd datetime ;

SET @DateBegin	='2019-07-15 08:00:00.000';
SET @DateEnd	='2019-07-25 20:00:00.000';

;with cte as (
	select @DateBegin as dt_start, dateadd(hour, case when datepart(hour, @DateBegin) in (8,20) then 8 else 4 end, @DateBegin) as dt_finish

	union all

	select
		cte.dt_finish as dt_start
		,dateadd(hour, case when datepart(hour, cte.dt_finish) in (8,20) then 8 else 4 end, cte.dt_finish) as dt_finish
	from cte
	where cte.dt_finish < @DateEnd
)
select * from cte