Newbie holding questions

I am getting ready to start IFR training next week, and there are a
couple of questions regarding holding that I have been pondering.
(Searched through here already, but couldn't find the answers...at
least spelled out for me

When being assigned a hold, will ATC usually assign an altitude as
well? None of the samples I've seen in the FAA text or the AIM mention
an altitude in a clearance, and it made me wonder what happens when a
large volume of aircraft are holding at a busy airport waiting to land.
Do they "stack 'em and rack 'em" (like it looked the two times I've
flown into LHR), or do they have them hold all over the place (which I
observed on the BOS Airport Monitor a few weeks ago when that snowstorm
rolled through.)

Secondly, what happens there is a holding pattern used to enter an
approach to landing, and there are multiple aircraft landing? For
example, if there were several planes flying into Charlottesville (CHO)
from the east (GVE VOR, 17 nm), wouldn't everyone be entering the
holding pattern at 3300', thus creating a collision hazard? Or would
ATC only clear one aircraft at a time for the approach? Which doesn't
seem to do much for volume, but normally isn't an issue at a place like
CHO unless there is a football game....

Thanks in advance.

JohnK
PP-ASEL, IA student