SUM over datedifference?

Lets say I have the following records with column datetime

dtTime	quantity
2020-12-10 19:21:52.293	1
2020-12-10 19:21:52.323	2
2020-12-10 19:21:53.293	1
2020-12-10 19:21:58.293	1
2020-12-10 19:21:59.193	1

How can I make this to return rows that are in range of let’s say 1.5second of eachother in the same rownumber? So sum the group by over:

Row_Number	dtTime	qauntity
1	2020-12-10 19:21:52.293	1
1	2020-12-10 19:21:52.323	2
1	2020-12-10 19:21:53.293	1
2	2020-12-10 19:21:58.293	1
2	2020-12-10 19:21:59.193	1

So desired sum result:

Row	quantity
1	4
2	2

To clarify further based on the comments: The situation is that there is a trigger, which will insert 10 to 20 records. The difference between dtTime of record 1 and record 20 will be MAX 2 seconds. The time interval between trigger 1 and trigger 2 will always be bigger than this 2 seconds. I hope this claries the question on how to group!

Answer

This is a type of gaps-and-islands problem. Use lag() and a cumulative sum to define the groups:

select min(dttime), max(dttime), count(*)
from (select t.*,
             sum(case when prev_dttime > dateadd(millisecond, -1500, dttime) then 0 else 1 end) over (order by dttime) as grp
      from (select t.*,
                   lag(dttime) over (order by dttime) as prev_dttime 
            from t
           ) t
     ) t
group by grp;

Here is a db<>fiddle.