Hello, I'm still breastfeeding my 9 1/2 month...

Hello, I'm still breastfeeding my 9 1/2 month old 3 times a day after her solid meals. I sometimes wonder if she is getting enough milk as the required amount is 600ml a day for a baby of Lucy's age. I'm worried as I know my milk supply would drop to adjust to demand as she feeds less frequently now. Does anybody else who is breastfeeding at this age supplement or do top ups with formula? I have never fed Lucy formula and she won't even take expressed breastmilk from a bottle. :(