The answer is simple: you need
one open seat for every butt in your traveling party
There is no way to guarantee that while non-reving you'll be able to get on
any flight you want. Delays, cancellations, mechanicals can all contribute to a flight that HAD been open, suddenly being oversold or revenue standybys showing up to travel. Or, conversely, a previously oversold flight may be able to accomodate you because of misconnects from other flights. I have been bumped off many previously wide-open flights, and I've been seated on many previously oversold flights.
Watch loads, watch weather, and know that there's just no guarantees. Flight loads are heavier than
ever now, and it's a different world now that it was even when I started in 1995. It
is possible to go many places, you just have to be flexible and maybe be willing to go through another hub, or be patient and sit and watch a couple flights go without you.