It is the worst possible idea to start at 8000.
Flags above 7999 don't save. (I actually don't remember the exact cutoff point, but I do know that flag 9999 is really not going to save. Probably).
You should start at 4001.
Use flag 4000, and drowning will crash the game.
Everything below 4000 is used for something, but there are also lots of gaps between Pixel's own flags. So there are plenty of flags below 4000 that are safe.
Everything from 4001 to 7999 is free for your own use though.
EDIT: I just checked Noxid's documentation on ASM, and yes, there are 8000 flags.
0049DDA0 EventFlags x 03E8, 1 bit each so 8000 flags.
Now, flags start at 0 and end at 7999. Notice that because flags don't start at 1, you cannot really use flag 8000.
Flag 8000 will still work in game because it's a RAM offset.
However, it will not save to Profile.dat, which is an important feature of flags.
I suggest that you don't use flags below 100. Definitely do not use flags 0-63 because those are actually skipflags that don't get saved anyway (I don't remember, but I think this is true).
You can't have more than 64 skipflags.
You can't have more than 8000 - 64 = 7936 flags saved to Profile.dat.