In California , when a married couple are receiving regular social security and SSI ?

SSI never pays your rent or anything else.  You receive the money and you pay your rent, bills, etc. from it.  The maximum would be when your money runs out.  If you are smart, you won't spend it all.  It would be a very stressful life.

SSI (a federal welfare program) doesn't pay rent and it never has.  You pay your bills out of your monthly social security and SSI income.  If your rent gets so high that your income doesn't cover it and other necessities then you move when you find a place you can afford - or you get roommates.

The maximum federal SSI benefit is the same in all states.  In 2020 it is $783 for an individual and $1175 for a couple.  If you receive more than that amount then you are receiving a supplement from the state of California.  Not all states supplement SSI.  Of those states which do the amount of the supplement varies considerably - some as low as $10 a month.

SSI is reduced by the receipt of any other income; e.g. your social security benefit.

I was a social security claims rep for 32 yrs.