For those who still check this blog post when maxing their cards, I've made a sheet on how to know the amount of stats and number of arcanas still needed:
It's easier
So... for those who wants to max their GLRs... First, a bit of an ado...
It is now common knowledge that 150.000 is the max stat of GLRs. But what is probably not well known is that, GLR defy common rules when it comes to computing their possible max stats. As it can be seen in this comment. You can also use this thread as a basis.
And while I was maxing my GLR True Wolf Fenrir's stats, I found out that GLRs have almost a set stat gain per level. (Only few levels differ, but most has the same stat gain.)
While this process of maxing your GLR's stats maybe a bit tedious, it is at least a bit cheaper than wild guessing how much ATK/DEF arcanas to be added. Wild guessing may result in you spending more rare medals than necessary, so... ¯\_(ツ)_/¯
To know which stats to max, use this article for reference. It also depends on your usage of the card. Like if it's an AoE but you will use it in your defense team, then it is advisable to also max the defense stat.
Requirements
Aside from N cards, gems, and rare medals,
 Patience  like lots of it (hahaha!)
Steps
This process is better to use if you did not take note of your card's stats at Level 1. Otherwise, you can skip this and proceed directly to #Alternative Way.
 [Optional] Sell your fountains (if you have it built), to at least ensure not to trigger Great Success when upgrading.
 Take note of the stats upon awakening. (SC doesn't matter, but you can still take note of it so you would be able to compute the max SC.)
 Take note of the amount of arcanas used after every level up.
 Take note of the stats after every level up.
 Compute the stat gain for every level up. I recommend doing this up to level 5 or 6 to be a bit sure on the stat gain.
Do the Maths
For an example of how to compute, please refer to this blog Calculating Arcanas Needed to Max GLR Stats.
 Stat Gain

current level's stats  stats from arcana added  previous level's stats = stat gain
 Approx Max Stats

Level 1 stats + ( Stat Gain * 109 ) = approx max stats
The 109 is the number of times the card will level up. So if there is a different stat gain computed, consider that in your formula too and change the number 109 as necessary.
For conservative purposes, you can directly use the lesser between the computed increases per stats.
 Approx amount of arcanas and rare medals

( 150.000  approx max stats ) / 50 = amount of +50 stat arcanas

amount of arcanas * 180 = amount of rare medals
From here, you can compute the remaining amount of arcanas still needed to max the stats. Just deduct the amount of arcanas you already used from the total amount of arcanas needed that you have computed.
Alternative way to know the amount of arcana still needed:
150.000  approx max stats  total stats from arcanas already added
Then divide anything you got by 50. That will be the amount of arcanas you still need to max your GLR.
Alternative Way
I'm including this alternative way since I've just come to know the other way of knowing total stat gain from level up.
 Formula

Awoken Card List stats  Collection Page stats = level up total stat gain
 Awoken Card List (ACL) stats  the stats you see when you check a certain card on the Awoken Card List
 Collection Page (CP) stats  the stats you see when you check a certain card in the Collection Page (Menu > Collection > Card). For you to see the collection page stats, the card (or the specific evolution of that card) needs to be unlocked. Other way to see the collection page stats is to check that card's wiki page. The first of the stats you always see here are the collection page stats.
For example; the stats in the red box are Karma's collection page GLR stats.
Continuing with the Karma example:
Stats  CP Stats  ACL Stats  Total Stat Gain  Stat Gain per Level Up 

Attack  55999  75999  20000  183.49 
Defense  44999  69999  25000  229.36 
Soldiers  52999  68999  16000  146.79 
Stat gain per level up is computed by dividing the Total Stat Gain by 109. And as explained above, 109 is the number of times a GLR card will level up.
So, how true is above computed stat gains? Well, thanks to Vei's, this is Karma's maths: https://cdn.discordapp.com/attachments/343946265626935308/353309239327981570/Karma_Awoke_and_Level_Info.jpg
So with this info,
Stats  Level 1 Awoken Stats  Total Stat Gain  Approx Max Stats (w/o Arcana)  Needed Stats (from Arcana to Max) 

Attack  70997  20000  90997  59003 
Defense  58497  25000  83497  66503 
Soldiers  67997  16000  83997  N/A 
Knowing the needed stats from arcana to max can now help you to determine how many arcanas and how much rare medals are you going to need.
Stat Gain List
Direct Link: https://docs.google.com/spreadsheets/d/1CT41xSuHyibfDSHQOyON4DkCPRlwW2WCy_ag6Pb76z4/edit?usp=sharing
So many maths, I know and I'm sorry....
https://discordapp.com/api/emojis/337788491784257537.png