I'd published a couple books years ago and the publisher's benchmark was 2000-4000 copies (depending on variables within their business) over any period of time was a commercial success. If you've done 300 in the first few months it seems fairly likely you'll get there. That said, the biggest bar to securing more writing opportunities for technical books is more about proving you can write consistently, so as a second time author you'll already be well ahead of the game. Technical publishers know the business and where there's enough interest and little enough competition to make money, so let them guide you on interesting areas to publish in and you can focus on great writing.