In Nop_Download, I think there should be another field added to that table named "MaxNumDownloads" that will let the administrator set the maximum number of downloads for a specific user. Another table would need to be created, something like Nop_Download_User and the fields would look something like this:
CREATE TABLE [dbo].[Nop_Download_Customer] (
[DownloadCustomerID] [int] IDENTITY(1,1) NOT NULL,
[DownloadID] [int] NOT NULL,
[CustomerID] [int] NOT NULL,
[DownloadIsPaidFor] [bit] NOT NULL DEFAULT(1),
[DownloadIsFree] [bit] NOT NULL DEFAULT(0),
[NumDownloads] [int] NOT NULL DEFAULT(1),
CONSTRAINT [Nop_Download_User_PK] PRIMARY KEY CLUSTERED
(
[DownloadUserID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
) ON [PRIMARY]
ALTER TABLE [dbo].[Nop_Download_Customer] WITH CHECK ADD CONSTRAINT [FK_Nop_Customer_Nop_Download_Customer] FOREIGN KEY([CustomerID])
REFERENCES [dbo].[Nop_Customer] ([CustomerId])
ALTER TABLE [dbo].[Nop_Download_Customer] WITH CHECK ADD CONSTRAINT [FK_Nop_Download_Nop_Download_Customer] FOREIGN KEY([DownloadID])
REFERENCES [dbo].[Nop_Download] ([DownloadID])
The value set in [MaxNumDownloads] in Nop_Download would determine if the user can still download this item when [NumDownloads] reaches the same value of [MaxNumDownloads].
[DownloadIsPaidFor] is just a flag that lets administrators control whether the item can be downloaded if it has not been paid for.
[DownloadIsFree] is a flag that will let the user download the file without any further prompting for payment.
