how big can you go with softboxes before you need to think about more than 1 speedlite?
Interesting question, and if you get replies from 10 different photographers then you'll probably get 20 different answers
It isn't really about power with modern digital cameras, you can usually crank up the ISO enough to compensate for low power, it's really about
Flashgun design
Softbox design
Softbox quality
What the softbox is used for
And different quality standards.
Flashgun design. If the flashgun is bare bulb, i.e. it doesn't have a fixed reflector, then it can even light really large softboxes well, allowing for the limited power. Assuming though that it has a fixed reflector, all that you'll get is a pretty uneven area of light that is thrown forwards, onto the softbox diffuser. If you can zoom the flashgun to a very wide angle then some of the light may also hit the walls of the softbox (which it will do well with a studio flash or a bare bulb flashgun) so the light distribution will be much better. Adding a diffuser to the flashgun will help to some extent, as will tilting the flash so that it's aimed at one wall instead of at the diffuser.
Softbox design. If the softbox is well-designed and deep then the light from the flashgun will travel further before it hits the diffuser, and will produce a better lit diffuser. But most cheap softboxes are very shallow. If the softbox has both inner and outer diffusers then again the light will be diffused and spread by the rear diffuser before reaching the front diffuser.
Softbox quality. If the softbox diffusers are thick and do their job properly then this will minimise the inevitable hotspot and do a better job, but most softboxes have terrible, inadequate diffusers. You generally need to spend quite a lot to get one that has been properly designed, most are just copies of copies of copies and the manufacturers have usually used the cheapest diffusion material possible.
What the softbox is used for can make a big difference. For example if it's used very, very close for quality critical work such as high end product photography then it needs to be right, and any lack of eveness in the lighting will ruin the shot. But if it's used for say family portraits, and probably placed too far away to act as a softbox anyway, then it doesn't make a lot of difference is the centre of the softbox front diffuser is a lot brighter than the edges.
And talking about
different quality standards will just get me into trouble, but you can work it out for yourself
I would say that for most people, most of the time, 60 x 60cm softbox is probably the maximum for half way reasonable quality, other people will have different views.
