I think generally folks that are adding more fish than 1 or 2 at a time have multiple quarantine tanks that are independent of each other (not connected to a common sump, etc) that way if one fish has a catastrophic infection the others don't get it.
Yes
Generally though it is recommended to slowly add fish to your DT because the biofilter often has to catch up to the new bioload.
yes
What a "safe" number is probably depends on a number of factors.
In the past ThRoewer has said that with Uronema infections all corals and inverts need to be culled or that display has to remain an all coral tank (no fish ever). I have found nothing that contradicts this, honestly. It's pretty scary to be in that situation.
Which is why unless one has a robust quarantine protocol, I advocate no chromis