I would guess over a thousand, many people used answer bank and other internet search engines, plus groups of people who may send more than 1 entry.
I too answered all questions correctly - disappointed, that it appears that only one winner was chosen from email entries.
I found out that they had about 52 entries by letter. All they would say is that they also had "thousands" of emailed entries.
I can only really speculate that they must have given more weight to a letter entry as they would realise that emailed entries can be rapidly duplicated by senders for multi-attempts at winning the prizes.
An emailed entry would in many cases merely have been copied from the many internet groups that were publishing the answers to this competition.