I know this is a pretty old thread, but there’s a LOT of unanswered threads about custom alert buttons.
Here’s my solution to this problem. I couldn’t find the answer anywhere so I just messed around with it.
Turns out you can make as many as you want. But only 1 of them can be colored red (destructive)
If you try to make more than 1 destructive it only makes the last one that’s destructive actually red.
This also prevents you from being restricted to what you manually create in the Thunkable editor screen.
Also there is a difference between iOS a d Andriod.
From thunkables website for DnD
"On iOS, the Alert can show more than 3 Buttons.
On Android, the Alert can show up to 3 buttons. If more than 3 buttons are specified, the first 3 will be shown. "