I been in the Times Crossword Club since it launched, and I've always presumed that only one entry is allowed. But I see what you mean, do new entries overwrite previous entries, which are then " collected " at the closing date.
I presume its the former but nothing is mentioned on the help pages.
You'll find " contact us " details in the Explore Crossword Club section ( although you often don't get any answer to your query)and there's always the bulletin board to ask other members.