All my answers go with the disclaimer of "in my opinion...your personal experiences can and will vary..."
Depends. Do you mean "have too many plots planned in your head" or do you mean "have too many plots thrown at the PCs"
(the first one is just referring to plans, the other is referring to actual implementation of any/all planned plots in to the game).
If you have too many thrown at the PCs, you'll possibly overwhelm them.
If you mean plots that you have planned, then, no, never too many. Because you never know (in a sandbox style) where the PCs will end up or what will interest them, so you need ideas in the back of your mind in case you're thrown for a creative curve-ball. And any ideas not used can be used for another campaign in some form or another...
I generally find that players hit a mental limit after some point - they'll either not have interest or not have attention span to follow too many plots (part of this is that we play somewhat infrequently, so take that comment with that fact in mind). But I tend to find 1-2 "active" plots that the PCs (and players) are most interested in, with 2-3 "background plots" planted as seeds so that they can be ready to come in to focus when the PCs have resolved one current active plot and ready for another one.
Yes... smaller scale things are less consuming. Large scale plots end up with all sorts of smaller steps. you'll reach a point where the players/PCs will ignore other things in order to advance one plot over the other (in terms of time priority) - once you hit that saturation point, no matter how many 'cool plots you have' it'll be wasted since the players/pcs won't have the time to think them through.
personally -- i never put a plot in stasis until the pcs get involved; the world is evolving.
the one caveat here is "is there even anything interesting if the plot does evolve" ? (ie someone becomes a more powerful threat than if they stopped it at its conception, or will there be a new ruler, etc) If the answer is no (i.e one gang leader vs another, and neither is any worse than the other), then i'd probably not even introduce the plot in the first place since its evolution has little to actually add to the dynamic setting and so the players action or inaction has no real effect.
(i'm tired, so apologies if the above is a little incoherent)