Wow, that many people on a track? I’ve been in some crowded races but that goes beyond belief.
As for starting, if you expect to be among the top 10-15 or so depending on the size of the track, I would say definitely fight for position on the line. As for where on the line to get, in something that crowded, I’d probably want to be at least 2/3 of the way out (outside of lane 6 on a 9 lane track, for example) in order to avoid getting boxed in. While this may lead to spending some time running in lane 2 or, in a race of that size, 3 or 4 for a while, that’s better than getting boxed in, especially with that many people.
Also of note, don’t be afraid to get physical. In a race of that size, elbows will fly and people will be pushed. Don’t be afraid to defend your space. Sometimes, a simple tap on someone’s elbow to let them know you are there will prevent them from cutting you off or maybe even give you space to move by. Sometimes, you have to do a bit of pushing or you may end up getting squeezed so tight that you end up face down on the track. That’s not something you want to do with that many people behind you, especially if any of them are wearing spikes.