I'm up to two now on my chain and I bought another three as I know I'll be breaking my chain again soon cause it's what I'm really good at it seems.....
I had a about 5 or 6 on one of my chains at one stage,
only got rid of it because the whole drvetrain was worn but i wouldnt go past that i dont think it would be too strong :)
In my mind a powerlink is far stronger than a plain pin so you could have half a chain of power links without any bother. I've had at least 3 in a chain in the past. But once you start getting to 4 or 5 its probably cheaper to buy a new chain.
often use big and big
I always leave enough chain length to go big and big
Mind you, with the longer chain, you can't use use granny and anything smaller than 3 on the back, but sure never did anyway