How do people estimate how much to invest in one counter and also per transaction? Do you set aside different investment amounts for each category, like stocks, REITs, ETFs, etc?

I would like to invest but what considerations do you have when thinking of how much to invest in a particular counter, assuming your portfolio is zero now - no investments yet. For now, I’m just thinking about the commission charges, if I divide it out, each transaction minimally has to be at least about $8k for it to be “worth it”. Is this one way how you guys estimate how much to put into each counter and transaction too? And what other ways?