JohnK wrote:
When being assigned a hold, will ATC usually assign an altitude as
well?

If you're IFR, you *always* have an altitude assignment. Sometimes
it's a single altitude (maintain 3000), sometimes it's a limit (at or
above 4000). If a new altitude is not explicitly issued along with
the holding instructions, you last assigned altitude still applies.


what happens when a large volume of aircraft are holding at a busy
airport waiting to land.

A lot of fuel gets wasted :-)

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.)

In theory, they could have multiple aircraft holding at the same fix
at different altitudes. The one on the bottom is cleared to land, the
ones above get cleared down to lower altitudes, and new arrivals get
thrown onto the top of the stack. In practice, that never happens any
more. Flow control procedures at busy airports are supposed to ensure
that flights don't get cleared for takeoff until a slot is available
for them at their destination. Still, **** happens and sometimes you
end up with more flights arriving at once than can land, so you gotta
do something with them. Delaying vectors are more common than holding
stacks.

Secondly, what happens there is a holding pattern used to enter an
approach to landing, and there are multiple aircraft landing?

Virtually all real IFR traffic these days is given vectors to final.
None of this holding at the IAF **** unless you're in training, or out
in the boonies somewhere and there's no radar coverage.