Was Voldemort directly responsible for every Imperius victim?
Just as Voldemort was unlikely to be directly involved in a lot of the murders under his reign, so I think he probably didn’t use the Imperius curse except on really key people. (For example, ranking Ministry officials or members of the Order.)
This is something confirmed in interview by JKR:
Will Lord Voldemort get more “screentime” in the upcoming books?
JKR: You will see him again, but like most evil dictators, he prefers his henchmen to do his dirty work.
Most of the Imperius victims across the country were under the control of Death Eaters on Voldemort’s orders. Many of these Death Eaters would have been killed or injured in the battle of Hogwarts, which could easily break the curse. Those who survived were probably captured or scattered. Either way, the Imperius curses were probably released.
(And anybody under Voldemort’s direct control would have been released when he died.)
How many people can one individual control with Imperius?
I don’t think canon gives particular limits for the curse. It probably depends on the magical ability of the person casting the curse, and the degree of control over/resistance from the victim.
In Goblet of Fire, Barty Crouch Jr (posing as Moody) uses the Imperius curse on many students in quick succession, at no apparent cost. Assuming most magical folk have similarly low levels of resistance, it seems plausible a powerful witch or wizard could curse multiple victims without too much of a mental burden on themselves.
The degree of instruction probably matters as well. Micro-managing a second person’s life is probably quite draining, but keeping them controlled with an instruction of “go about your normal life, act as if nothing is wrong” until further notice probably isn’t.
For example, Malfoy keeps Madam Rosmerta under the curse for several months (at least from October 1996, the cursing of Katie Bell, to June 1997, the eve of Dumbledore’s death). Her instructions were probably fairly mild – poisoning a bottle of mead, attacking a single student, otherwise acting normally. (Although perhaps the curse needed refreshing every so often.)
Finally, I think we can assume that controlling somebody like Harry (an extremely capable wizard who could resist the control of Imperius) would be more of a burden than a more passive victim. I think it unlikely that a single person could control multiple victims who were putting up a good fight.
Could you get a chain of Imperius victims?
Yes. There is (at least) one example in the canon.
As stated above, Madam Rosmerta spends most of Deathly Hallows under the control of Draco Malfoy. She in turn places the curse on Katie Bell, so we have a chain Malfoy -> Rosmerta -> Bell.
It’s plausible that this was used in other ways, to infiltrate important organisations – for example, curse a single person at the Ministry, and have them curse their colleagues. However, I don’t think this is ever explicitly described in canon.