It doesn't really matter what you number them, as long as you follow these basic rules:
1) do NOT use numbers lower than 100, because head.tsc uses numbers lower than 100, and 100 is a good safe spot to start.
2) do NOT use ludicrously large numbers. I think anything over 1000 is iffy, and much past that and some ambiguous things might happen. How ambiguous? Nobody knows!
(yes, I know I used the word ambiguous wrong)
3) absolutely do NOT put the numbers out of order. This will cause some amusing and confusing glitches involving out of sequence events.
Also, for skipflags, I recommend using # that are under 20, and for regular flags use # between 100 and 1000 (and keep a list of what flags are used when and for what, it'll help prevent accidental reuse of an already set flag).
Personally, I have a little system set up for the numbering of my events.
events 100-199 are door/transport events
events 200-299 are npcs n' stuff
events 300-399 has something to do with bosses
400-499 is alternate events (for flags to jump to)
500-599 is random stuff
and you should probably never need any more than that, unless you make maps crazy big.