Max xp stacking

[Whirlzap]

It's 3x combo, thrasher, and 2x XP weeklong, making it a total of x9 xp if you're in mobs that give 1 xp per kill. With a double xp weekEND, it makes the total x18.

That is an incorrect interpretation of the stacking mechanics behind EXP elixirs.
The elixirs stack mutually exclusively, not collectively.
For example, the 3x Combo gives 3 times your base EXP, but what this means is that the elixir ITSELF gives only 2 times EXTRA EXP.

In calculating total EXP, you are over-counting the base EXP each time you add on a new elixir. To figure out correct total EXP, you need to total up base EXP along with total EXTRA EXP.
So basically, 3x Combo [+2x EXTRA EXP], 2x XP weeklong [+1x EXTRA EXP], and thrasher [+3x EXTRA EXP] total up to 6x EXTRA EXP. Add your base EXP to get a total of 7x EXP as the cap.
The double EXP weekend works over accumulations, so you can actually double 7x to get 14x EXP as your maximum.

In the past, there used to be an added EXP bonus if you ran in a full party. This bonus still worked as of Sewers cap, but it hasn't been tested for a long time, so I don't know if it's still in the stacking mechanics. Nevertheless, the bonus was a 1.3x boost, so a total of 7.3x EXP if this still works, thereby capping at 14.6x EXP on a double EXP weekend.

Thanks for the reply. For some reason I thought that the xp gain stacking was limited to 7x, with the exception of double xp weekend. Maybe that's just what was possible before Daily Blessings.

No, that's still the correct cap. Assume all elixir boosts are mutually exclusive (remove base EXP from each elixir when calculating stacking and then add it once at the end), except the double EXP weekend "boost", which is over total stacked EXP.