SQL Server 2017

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select * into Tickets 
from(
  values 
    (1 , null), 
    (2 , null),
    (3 , 123), 
    (4 , null),
    (5 , 124), 
    (6 , null),            
    (7 , null), 
    (8 , 123), 
    (9 , null), 
    (10, 123),            
    (11, null), 
    (12, null)
)t(TicketId, AssignedTo)
12 rows affected

select * into Empl
from(
  values 
    (123),
    (124),
    (125),
    (126),
    (127)
)t(EmployeeId)
5 rows affected

with cte as (
   select e.EmployeeId, count(t.TicketId) n
   from Empl e
   left join Tickets t  on t.AssignedTo = e.EmployeeId 
   group by e.EmployeeId
   having count(t.TicketId) < 5
   
   union all
   
   select EmployeeId, n+1
   from cte
   where n < 4
), EQueue as (
   select EmployeeId, n, row_number() over(order by n, EmployeeId) rn
   from cte
), TQueue as (
   select TicketId, row_number() over(order by TicketId) rn
   from Tickets
   where AssignedTo is null
)
select t.TicketId, e.EmployeeId AssignedTo
from EQueue e
join TQueue t on e.rn = t.rn


TicketId
AssignedTo


1
125

2
126

4
127

6
124

7
125

9
126

11
127

12
124


Warning: Null value is eliminated by an aggregate or other SET operation.