There are five different versions of Tixx's Infinarium, each one rewarding you with 100 Mystical Cogs when completed. Finishing all five grants you another 200, for a total of 700 cogs.
Each holiday mini pet requires 250 cogs to craft. This means that you can only craft two.
What are the remaining 200 cogs good for?
Best Answer
For one day, there was a bug where mystic cogs suffered from Dungeon Diminishing Returns. I.e., if you completed another dungeon, then ran Tixx's dungeon, you would get less than 200 mystic cogs.
As far as I'm aware, this was Anet's 'fix' for those adversely affected, so that everyone has enough mystic cogs to make exactly 2 minis, as 200 extra cogs is not enough for someone not affected by the bug to get another one.
To the best of my knowledge, there is no other use for the mystic cogs --- at least, this year!