I'm in the UK.

My letter is C5 in size.

It weighs no more than 50g

It is about 3mm thick.

How MANY stamp should I put on it?

Only Second class stamps.

1

2

If it's national post, only 1 stamp will be needed, size of that looks like a standard size envelope. If it's abroad via airmail they will weigh it in the PO and far as i know 2nd class stamps will not matter !!!

1 to go 2nd class. Larger than C5 would cost more.

I'm in the UK.

My letter is C5 in size.

It weighs no more than 50g

It is about 3mm thick.

How MANY stamp should I put on it?

Only Second class stamps.