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;