Depends on the individual and how they accrue their income. Someone with a six figure income, which includes investment income and income from selling their time, would fall under middle class but someone who makes that six figures from working alone would qualify as working class. It is not the amount that matters here, it is how that amount is generated.
You're middle class if you are applying your own labor to capital you own to produce your income.
You're capitalist class if you are renting other people's labor to apply to capital you own to produce your income.
(These aren't discrete categories; they might be better read with "if you are" replaced with "to the extent you are".)