SQL TSQL for Workers per Hour

Question

I have a log with fingerprint timestamps as follows:

Usr   TimeStamp
-------------------------
 1    2015-07-01 08:01:00
 2    2015-07-01 08:05:00
 3    2015-07-01 08:07:00
 1    2015-07-01 10:05:00
 3    2015-07-01 11:00:00
 1    2015-07-01 12:01:00
 2    2015-07-01 13:03:00
 2    2015-07-01 14:02:00
 1    2015-07-01 16:03:00
 2    2015-07-01 18:04:00

And I wish an output of workers per hour (rounding to nearest hour) The theoretical output should be:

 7:00  0
 8:00  3
 9:00  3
 10:00 2
 11:00 1
 12:00 2
 13:00 1
 14:00 2
 15:00 2
 16:00 1
 17:00 1
 18:00 0
 19:00 0

Can anyone think on how to approach this as SQL or if no other way, through TSQL?

Edit: The timestamps are logins and logouts of the different users. So at 8am 3 users logged in and the same 3 are still working at 9am. One of them leaves at 10am. etc

Answer 1

Here is my final working code:

create table tsts(id int, dates datetime)
insert tsts values
(1 ,   '2015-07-01 08:01:00'),
(2 ,   '2015-07-01 08:05:00'),
(3 ,   '2015-07-01 08:07:00'),
(1 ,   '2015-07-01 10:05:00'),
(3 ,   '2015-07-01 11:00:00'),
(1 ,   '2015-07-01 12:01:00'),
(2 ,   '2015-07-01 13:03:00'),
(2 ,   '2015-07-01 14:02:00'),
(1 ,   '2015-07-01 16:03:00'),
(2 ,   '2015-07-01 18:04:00')

select horas.hora, isnull(sum(math) over(order by horas.hora rows unbounded preceding),0) as Employees from

(
select 0 as hora union all
select 1 as hora union all
select 2 as hora union all
select 3 as hora union all
select 4 as hora union all
select 5 as hora union all
select 6 as hora union all
select 7 as hora union all
select 8 as hora union all
select 9 as hora union all
select 10 as hora union all
select 11 as hora union all
select 12 as hora union all
select 13 as hora union all
select 14 as hora union all
select 15 as hora union all
select 16 as hora union all
select 17 as hora union all
select 18 as hora union all
select 19 as hora union all
select 20 as hora union all
select 21 as hora union all
select 22 as hora union all
select 23
) as horas

left outer join 
(
select hora, sum(math) as math from 
    (
    select id, hora, iif(rowid%2 = 1,1,-1) math from 
        (
        select row_number() over (partition by id order by id, dates) as rowid, id, datepart(hh,dateadd(mi, 30, dates)) as hora from tsts 
        ) as Q1
    ) as Q2
group by hora
) as Q3 

on horas.hora = Q3.hora

SQL Fiddle