The amount of armor they can have converges. They can max out with armor up to a max of slightly less than 166.66% of the armor of the highest base armor Trundle. Warning: math below.
Trundle steals 40% of his targets armor, which is 2 fifths or 2 out of 5. If his target has 5 thirds or 5/3 of his armor (or 1 and 2 thirds of his armor), then he will steal a value equal to 2 thirds or 2/3 of his armor, leaving him with 1 and 2 thirds of his original armor.
If a Trundle steals more than 66% of his armor, he would not reach the amount of armor his target had. If a 100 armor Trundle steals from a 200 armor Trundle, the 100 armor Trundle ends up with 180 armor. So any overshooting of the 166% will go back down towards 166%.
The Trundles in the video have probably about 700 armor, so they should be capping somewhere around 1160, or about 166% of 700 (assuming that is their actual armor value).
TLDR: Read the first paragraph.