The supply will be constant after the halving no matter how many miners there are. There will be 12.5 bitcoins produced every ten minutes whether there are a billion miners or a million miners. Supply production will be constant, it will just be half of what it was before the halving.
Less than 3% of the daily Bitcoin volumes are taken up by the newly mined coins (looking at the transaction volumes in the major exchanges). So I am not sure whether there will be a sudden supply / demand inequality once the block halving occurs. And also, the block reward halving is still some 7 months away. A lot can happen before that.
For sure. If anything, the disruptions to supply (to the extent they occur) will more likely be caused by expectation of what should be happening to the price and the perception of what is happening with supply, as opposed to what is actually happening.