Congratulations, you are the 5th person to reach the limit. The previous 4 accounts had accidentally reached the limit when an app that they sync with went crazy and added duplicate tasks. It sounds like you may be the first person to legitimately reach our limits.
Toodledo was not designed to work with this much data per account. In fact, your browser would probably crash if you tried to load all 20,000 in one list. You will need to export your old completed tasks and then delete some of the oldest ones to make room for more.
I recommend exporting your completed tasks as a CSV file. You can open this in Excel and then always have access to your entire history of completed tasks even if they are not in Toodledo anymore.