Some of the cards in the game have limited edition parallel versions. These are, #/75 #/25, #/5 and #/1 (with the exception of Eternal Gold cards, which only have #/25, #/5 and #/1 versions available). The number of subjects that have these variations are:

Base: 288 Subjects
Tier 2 (Next Goal Wins): 64 Subjects
Tier 3 (Vibrant Velocity): 42 Subjects
Tier 4 (Master Magicians): 35 Subjects
Tier 5 (White Ice): 28 Subjects
Tier 6 (Inferno): 26 Subjects
Tier 7 (First Class): 14 Subjects
Tier 8 (Reign Supreme): 11 Subjects
Tier 9 (Eternal Gold): 2 Subject
Total Performers: 20 Subjects
Parallels are minted on a per card basis, not a per pack basis. This means that every pack opened has a chance of receiving a parallel, whilst stocks last. When a card is minted, if that card is one of the subjects that has a parallel variation still available, it has the following chance of being a parallel:

Base
#/75 Edition: 1.23%
#/25 Edition: 0.41%
#/5 Edition: 0.08%
#/1 Edition: 0.02%
 
Tier 2 (Next Goal Wins)
#/75 Edition: 4.86% 
#/25 Edition: 1.62%
#/5 Edition: 0.32%
#/1 Edition: 0.06%
 
Tier 3 (Vibrant Velocity)
#/75 Edition: 5.54%  
#/25 Edition: 1.85%
#/5 Edition: 0.37%
#/1 Edition: 0.07%
 
Tier 4 (Master Magicians)
#/75 Edition: 8.50%   
#/25 Edition: 2.83%
#/5 Edition: 0.57%
#/1 Edition: 0.11%
 
Tier 5 (White Ice)
#/75 Edition: 10.49%    
#/25 Edition: 3.50%
#/5 Edition: 0.70%
#/1 Edition: 0.14%
 
Tier 6 (Inferno)
#/75 Edition: 12.32% 
#/25 Edition: 4.11%
#/5 Edition: 0.82%
#/1 Edition: 0.16%
 
Tier 7 (First Class)
#/75 Edition: 15.81% 
#/25 Edition: 5.27%
#/5 Edition: 1.05%
#/1 Edition: 0.21%
 
Tier 8 (Reign Supreme)
#/75 Edition: 16.21%  
#/25 Edition: 5.40%
#/5 Edition: 1.08%
#/1 Edition: 0.22%
 
Tier 9 (Eternal Gold)
#/25 Edition: 6.47%  
#/5 Edition: 1.29%
#/1 Edition: 0.26%
 
Total Performers:
#/5 Edition: 1.56%
#/1 Edition: 0.31%
If a pack contains more than 1 card, this multiplies your chances of receiving a parallel. Each draw of a card is unique and isolated from all other draws. These calculations have been formulated based on total expected digital counts of each subject across all tiers of rarity, using 23/24 figures as a guide.