public partial class ExpSections
{
    public ExpSections()
    {
        ExpRemarks = new HashSet<ExpRemarks>();
    }
    public int ExpSectionId { get; set; }
    public int ExpMainId { get; set; }
    public string Name { get; set; }
    public string ExpertName { get; set; }
    public string ImageName { get; set; }

    public ExpMains ExpMain { get; set; }
    public ICollection<ExpRemarks> ExpRemarks { get; set; }
}

public partial class ExpRemarks
    {
        public int ExpRemarkId { get; set; }
        public int ExpSectionId { get; set; }
        public string Number { get; set; }
        public string Question { get; set; }
        public string Answer { get; set; }
        public string Check { get; set; }

        public ExpSections ExpSection { get; set; }
    }

public IActionResult Method(int id)
{
     ExpSections expSections = _bd.ExpSections.SingleOrDefault(m=>m.ExpSectionId == id);

Console.WriteLine(expSection.ExpRemark.Count());

_bd.ExpSections.Include(x => x.ExpRemarks)
               .SingleOrDefault(m=>m.ExpSectionId == id);

public virtual ICollection<ExpRemarks> ExpRemarks { get; set; }