I don't know the average for everyone, but I do have a live store running 4000-5000 products with over 50 categories. In performance testing I've done 10,000 products without any noticeable performance drop, I've heard people say they are running 15,000 products without issue. It's difficult to say what the max should be, it all depends on you server/DB/bandwidth spec. plus how many orders your taking, plus how many visitors you have, plus how many user accounts you many have, plus any other stuff running within the same DNN and any other software on your server...etc...
But as for the software I wouldn't worry about your store size, unless you have a massive product base. In which case you should ask yourself I'm I doing right to use a free cart rather than paying for something that is supported and where I can ask for help is things go wrong!
But as for the software I wouldn't worry about your store size, unless you have a massive product base. In which case you should ask yourself I'm I doing right to use a free cart rather than paying for something that is supported and where I can ask for help is things go wrong!