Delphine,
How did you determine such small numbers? Astrid surely doesnt impose any cap on numbers of completed tasks. What you might experience is that Astrid only syncs active tasks.
So when you are starting Astrid from scratch after a clean install, be sure to always import the latest backup to have all of your completed tasks in the database. Then sync after that, and Astrid will recognize during sync, when you e.g. reactivate an old task from the website, will reactivate it too with all notes and list-tags intact.
Just some number from me: I sync with Producteev too and have about 600 tasks in my shopping list which I recycle on a regular basis.
Arne Jans