If you just want people to call you a bitcoin investor, then by owning 100 satoshis, you are already a bitcoin investor.
For me, this is not an investment, it's just the first step, an attempt to create something, it's definitely not an investment. I see on the forum they create topics asking if it is possible to start buying bitcoin with $ 30 a month, in fact, you can start buying with any amount, but you will not become an investor from this, it will take too much time to create some kind of more or less significant investment.
I don’t want to name any amount, it will be different for everyone, but in the end you need to be realistic and understand what the investment can lead to if bitcoin reaches, for example, the price of 100k. Therefore, everyone can calculate the prospects for their investments.
if owning bitcoin does not make you an investor, then if someone wants to become a bitcoin investor, what are the necessary and sufficient conditions? can you tell me what is the definition of an investor? and in case OP's friend owns 0.8 BTC, is he not a bitcoin investor?
I also don't want to mention the amount invested because everyone has a different economy, and I think it has nothing to do with being an investor.