Q: How many things I have to collect in Necropolis Halls?
A: Thankfully there aren't many. You only have to collect 17 chests and 1 Lyrium chest.

Q: When should I start progressing?
A: This is up to you, however keep in mind paths to some chests are locked behind some quests.

Q: What quests lock paths to these chests?
A: Here's the list (in no particular order):
1. Hauntings: From Beyond
2. Restless Spirit.
3. Walking the Graves

Location:
In the same room where the bell is found.

Solution:
Chest is right at this location here.

Location:
From previous location, go North-East.

Solution:
Interact with the open stone coffin.
A puzzle will start in the previous room. Follow the purple flames and interact with the bones. There is a total of 5.
Return to the coffin again and take the wisp to this pedestal.
Chests are behind the gate.

Location:
From previous location, go South-East.

Solution:
Go through the same door when you did in Emmrich's recruitment mission.
Chest is right there.

Location:
You will need to have Taash recruited in order to get access to this chest.
From previous chest, keep going South.

Solution:
Use Emmrich's ability to free the wisp.
Solve the buzzle by lighting up the brazziers with Taash's ability by following the wisp path.
Take the wisp back outside in the Cascades to this pedestal. Chest is right behind the gate.

Location:
From previous location, get here.

Solution:
Use Spite's ability to bring a bridge from here.
Cross the bridge and jump on these beams down.
Take the slide down and the chest is just beneath.

Location:
Go South in The Path of Sighs.

Solution:
It's left to the door that goes back to the bell.

Location:
Go up top, where you destroyed the venatori barrages for the bell.

Solution:
Chest is right at the very end of the path.

Location:
Make sure you have "Restless Spirit" quest active.
Go where the bell stands.

Solution:
Start making progress for the quest.
Chest is behind this door.

Location:
Make sure "Restless Spirit" quest has been completed first.
Travel to the newly unlocked path, North-West.

Solution:
Chest is on the right mid-way through the corridor.

Location:
Make sure you complete "Walking The Graves" quest.
Once the quest ends, go here.

Solution:
Interact with the bell and interact with the bells in the order the wisp goes.
Return to the initial bell to get the chest.

Location:
From previous chest, go towards the middle of the graveyard.

Solution:
Go through this now opened portal.
Chest is right in front of you.

Location:
From previous chest, go South-West to here.

Solution:
Interact with this door. Then use Emmrich's ability to free the wisp.
Take the wisp at the end of the path here. Chest is behind the gate.

Location:
Go to the Vault of the Beloved (longest corridor in Necropolis Halls).

Solution:
Use the lever to open the gate, then interact with the skeleton sitting on the chest.
Light up the candles that asks you to in the corridor (6 candles in total) then return to claim the chest prize.

Location:
Make sure you have completed "Pinnacle of its kind" quest.
In the dragon arena, get to here.

Solution:
Chest is right there.

Location:
From previous chest, go to the opposite side of the arena.

Solution:
Chest is right there.

Location:
Make sure you have completed "Hauntings: From Beyond" quest first.
Go up North of the map.

Solution:
Defeat the Haunt, then travel to a different place in Thedas then return to Necropolis Halls
Go even northern of the map and the chest is now on the stairs here.

Location:
Go back to the longest corridor in Necropolis Halls (requires Chest #14 to have been collected).

Solution:
At the end of the corridor in South-West, go through this portal.
Chest is right in front of you.

Hope this guide was helpful and you've achieved Necropolis Halls completion :)
Always open to feedback!