"wonder if/when they next encounter a planetary body"
Given their orbit, the only planet they're likely to encounter in the next few thousand years is likely to be Mars, and when they do they'll burn up leaving nothing more than a tiny bit of soot.
Still, you seem to be worried, so lets do some maths:
Obviously we can't measure the size of all of space, but then we're only really interested in the bit around us, so lets imagine the disk of the solar system, out to Mars orbit, and because most stuff orbits in the same plane, we can imagine it as a disc only as thick as the Earth (12x10^6m).
So, radius of Mars orbit (ish, it's really an ellipse) is 228 million km, or 228x10^9m
The area of our disc would then be 1.63x10^23 m^2, and the total volume would be 3.7x10^31 cubic metres (that's 37000000000000000000000000000000 cubic metres, more or less).
If we assume that each one of these cube sats is one cubic metre (they're smaller than that, but it's close enough), I hope you can see that 1 into 3.7x10^31 is a tiny, tiny fraction. And this isn't even the whole solar system, it's just a thin slice of the area out as far as Mars.
Space really is quite large.