How many languages can you think in?

Does British English and American English count as two separate languages?

One. And barely that sometoimes
There are a couple of things that I can automatically come up with in other languages without taking deliberate steps to translate, but just like saying thanks or excuse me or "I don't speak french" in french. Not exactly thinking in another language.
Although saying "I speak french" in french does help to one up smug kids when they start boasting their language skills to the ski instructor in english. I can't follow it up to save my life, but for one glorious moment I outdid them and that was sweet.

3. English, Hindi and Marathi.

What is your default thinking language? What age did you learn them?

Learnt English at age 3 (even though got fluent many years later). Marathi is my mother tongue. Learnt Hindi at age 7.

My default language for think was Marathi until around age 13 when I started making conscious efforts to think in English (in order to improve).

English and Cantonese, my mandarin is quite poor so I have to translate it back to English in my head.

3.

English, Japanese and Bengali.

I grew up as a multilingual fluent in English and Bengali as well as understanding completely but not being able to speak/write/read Hindi and Urdu.

I grew up watching Japanese animation even though I didn't understand the language and relied solely on the subtitles. In primary school I studied French; in high school I studied French and Spanish; and, at university I studied Japanese, French and Spanish. My level of English greatly decreased at university even though it was my best subject at high school and I was always considered to be well-spoken.

I spent a year studying in Tokyo during my undergraduate degree. During this time, my thinking voice shifted to Japanese and my general level of English and Bengali decreased so much so that I struggled communicating with family and friends back home.

As soon as I graduated university, I went to Japan for work where I stayed for a year. I had many foreign colleagues and spoke quite a bit of English at this time, so my level of English didn't decline like it had done during my previous stay in Japan.

These days I'm situated in the UK and as a result I mostly think in English. In Japan, my thinking voice changes to Japanese. I used to think in Bengali when I was a child until the age of around 14 or 15.

Since my year studying in Japan during my undergraduate degree, my dreams have shifted to being in Japanese which they are even now.