There is not a single answer to this question. I think it's best to work under "on Tick", check the current number of orders and do something depending on their number... if this is the idea. If the EA is restarted, it should continue as if it was not restarted. Note that "once per trade/order" uses temporary memory which is reset when the EA is restarted. Also note the option inside.
Otherwise you can work under "on Trade", but each Trade signal happens once, which can be good or bad.