There are approximately 3700 stocks. ETFs are not quite relevant because the majority does not comply with MiFID regulations and cannot be offered within the EU.
Two thousand stocks can effectively be considered as “thousands”. My guess is that Freetrade will switch on most large/mid cap and liquid stocks, then expand depending on demand. Also, Adam may have referred to other markets, such as EU, Canada, Switzerland, HK, Japan and others; in fact, European stocks are already on track in the next six months, followed by Asia afterwards. You can access more details here.
There is absolutely no point in offering every single stock for the sake of it if there are additional costs attached, otherwise, it will not be a sustainable commission-free service.