What are Vivaldi's limits?
How many pages maximum?
How many tabs maximum per page?
(Does the maximum tabs per page change with many pages open?)
The question is confusing, sorry.
One tab contain one site page.
Did you mean tabs per Window or maybe per Stack?
I think the limit about tabs/pages is correlated to how much ram you can use in your OS and ram usage from any tab, plus the extensions - which also take ram.
On win, for example, I can "safely" open more than 50 loaded tabs with 8 extensions on, and everything start to go slow when almost the ram - 8gb - is in use.