It depends on what you wnat to achieve....I normally have about 50-60 Superior and countless lesser poitions...but there is no rule about a fixed number as the goal you want to achieve is not clear...

I do 10 runs every day on floor 20.
today it's blue, I have 2726 small, 1711 medium and 35 giga in stock 

I want to have enough in stock at all times to do a fusion.  

Well, as you do not know, how the Fusion will be arranged...it is quite a simple calculation:

Add the potions you need to ascend 16 rare champs to 4* 

Add the potions you need to ascend 4 epic champs to 5*

As you do not know the affinity, you need the amount of potions for every affinity.

A simple search via google leads to a table on reddit...

From this: Rare: 10/4 (Lesser/Greater) Affinity + 5/3 Arcane Potions -> 160/64 Affinity + 80/48 Arcane

Epics: 20/6 (Greater/Superior) Affinity + 15/3 Arcane Potions -> 80/24 Affinity + 60/12 Arcane

Together: 

160/144/24 Potions of each Affinity

80/108/12 Arcane Potions

This is base maths... 

Hi! You may find this post on Reddit useful. It considers the number of Spirit potions one would need to successfully complete Hellicath Fusion: https://www.reddit.com/r/RaidShadowLegends/comments/v6u3un/resources_for_helicath_fusion_potions_food_and/

Basically, the same math applies across all Fusions with a similar pattern. 

Lots of colors 0_0

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