The number of tabs are actually the reflection of your computer's memory state. They just so appear to show up as tabs on the screen. How to portray them to the end user is up to the designers. As you're talking about Safari, let's focus on how Apple renders their user interface.
Apple's design philosophy
Arguably they've been utilizing Skeuomorphism. Just look at their "Notes" app - they do a great job making it look/feel like an actual piece of paper. So what do they represent tabs to be? Here are some possible assumptions:
1. Tabs are wrappers around page(s)
In this case, it' s natural to say 1枚.
2. Tabs are containers for page(s)
In this case, it's natural to say 1個.
Once Apple defines what tabs are supposed to portray, their localization team decides how they should be counted.
I have never counted tabs with my friends in my life, nor have I implemented such message for the Japanese. But I think 1個 or 1つ is appropriate. 1枚 sounds strange, probably because I think tabs are containers.
Or are tabs counted with generic つ that becomes 個 for numbers greater than 10, but the string interpolation can’t do Japanese numerals in its complexity?
I don't represent Apple, but they're probably capable of handling such case. So this shouldn't be the reason to avoid certain counters.
- 5枚 is valid
- 5個 is valid
- 5つ is valid
- 10枚 is valid
- 10個 is valid
- 10つ is invalid and probably replace with 10個