Dynamically Populating Multiple Gridview Data Based on Condition

I see your GridView is only showing the first row for each selected insurer .Try looping through all the related rows like this:

DataTable dt = new DataTable();
var security = rida.ReInsurer_Securities(ricode);

dt.Columns.AddRange(new DataColumn[3] 
{ 
    new DataColumn("Id", typeof(string)), 
    new DataColumn("Sub_ReInsurer", typeof(string)), 
    new DataColumn("RI_Share", typeof(string)) 
});

foreach (var s in security)
{
    foreach (var row in s.RelatedRows) // Adjust 'RelatedRows' according to your structure
    {
        string Id = row.Id;
        string Security = row.Sub_ReInsurer;
        string security_share = row.RI_Share;

        dt.Rows.Add(Id, Security, security_share);
    }
}

GridView1.DataSource = dt;
GridView1.DataBind();

This should ensure that all the rows for each selected checkbox are displayed. Give it a shot and let me know how it goes!

Hope this helps