MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table projects (Project_id int,	Employee_id int);

insert into projects (project_id, employee_id) values 
(1,	1),
(1,	2),
(2,	1),
(2,	1),
(2,	1);
select temp.project_id from
(select project_id, count(distinct employee_id)
  from projects
  group by project_id
  order by count(distinct employee_id) desc
  )temp
limit 1


project_id


1


select project_id from projects
group by project_id
having count(project_id)=
  (select count(project_id) from projects
   group by Project_id
   order by count(project_id) desc
   limit 1)


project_id


2