There isn't really a "safe" number of open seats because there are so many factors that could impact availability at the last minute. I can't speak to dl's policy but I can say that at my airline you may not have a purchased ticket for the same city pairs on the same day that you plan to nonrev. In other words, you may not hold positive space only to cancel when you decide that there are plenty of open seats and nonrev. At my airline, doing this results in instant loss of your travel benefits and possible termination. Check the rules carefully or you may face the same thing. You can list as a nonrev and LATER decide to buy a ticket because there was no way you were getting on but doing so ahead of time is a no-no.