SELECT dbo.Activities.id,
       dbo.Activities.PositionId,
       dbo.Activities.ResourcesId,
       dbo.Activities.Month,
       (
           SELECT COUNT(*) AS Repeats
           FROM   dbo.Activities AS a
           WHERE  (PositionId          = dbo.Activities.PositionId)
                  AND (ResourcesId     = dbo.Activities.ResourcesId)
       ) AS Repeats,
       dbo.Resources.Name
FROM   dbo.Activities
       INNER JOIN dbo.Resources
            ON  dbo.Activities.ResourcesId = dbo.Resources.id

Guid positionId = New Guid("E745EB39-88FB-40B8-84AD-01332D636D14");
var list = (from a in DataContext.Activities
    join r in DataContext.Resources
        on a.ResourcesId equals r.id
    select new Activity()
    {
        Id = a.id,
        PositionId = a.Position.id,
        ResourceId = r.id,
        ResourceName = r.Name,
        ApMonth = a.Month,
        IsEmpty = false,
        IsOverlapped = false,
        Repeats =
            DataContext.Activities.Where(x => x.PositionId == positionId)
                .Count(x => x.ResourcesId == r.id)
    }).ToList();