Low class millionaires are those with individual (not combined with spouse) net worth of less than $5m. So, a couple with combined net worth of less than $10m is considered low class millionaires.

Middle class millionaires - individual net worth of between $5m to $50m.
Upper class millionaires - individual net worth of between $51m and $200m.
High class millionaires - individual net worth of more than $200m.