A list item in SharePoint can have multiple attachments.  Below is a code snippet in c#.Net to add an attachment to a list item programmatically.
This example demonstrates how a file, uploaded through a file upload control is added as an attachment to a list item.

using (SPSite oSPsite = new SPSite(http: //website Url/))
{
     using (SPWeb oSPWeb = oSPsite.OpenWeb())
      {
           oSPWeb.AllowUnsafeUpdates = true;
 
            // Fetch the List
            SPList list = oSPWeb.Lists["MyList"];
 
            // Get the List item
            SPListItem listItem = list.GetItemById(1);
           
            // Get the Attachment collection
            SPAttachmentCollection attachmentCollection = listItem.Attachments;
 
            Stream attachmentStream;
            Byte[] attachmentContent;
 
             // Get the file from the file upload control
            if (this.fileDocAttach.HasFile)
            {
                     attachmentStream = this.fileDocAttach.PostedFile.InputStream;
                 
                     attachmentContent = new Byte[attachmentStream.Length];
 
                     attachmentStream.Read(attachmentContent, 0, (int)attachmentStream.Length);
 
                    attachmentStream.Close();
                    attachmentStream.Dispose();
 
                     // Add the file to the attachment collection
                     attachmentCollection.Add(this.fileDocAttach.FileName, attachmentContent);
            }
 
            // Update th list item
            listItem.Update();
 
            oSPWeb.AllowUnsafeUpdates = false;
       }
 }