I couldn't express at all, but definitely made enough milk. They don't necessarily correlate! Baby had a bottle of formula per day until 4 months, when we increased the amount formula in the day (out of choice). We also did formula top ups when the baby had jaundice (not on advice, again my own choice) until baby made their back birth weight.
We did formula from day 2; for a long time my baby would only take formula from my partner and not from me, which I hear is common for combination fed babies/pumped BM. To begin with the baby only needed 5-10ml formula, they take so little to begin with because they have tiny stomachs. We were using the pre-made bottles with sterile teats; my baby couldn't take the Tommee Tippee bottle until about 3 weeks (mouth too small maybe). I combination fed until 8 months, so we had time to see my baby adapt and change. If you do combination fed with formula or pumped milk, don't fall into a trap worrying your baby isn't getting enough milk because of what you can or can't pump, or because of the duration of breastfeeding sessions. If the baby is peeing, pooing, growing/gaining weight, they are getting enough from you.
The health visitor told me they don't advise giving a bottle until 4-6 weeks, but that in her experience, babies often take it more readily if it's introduced earlier. So make of that what you will!
Don't feel obliged to pump if you find you like breastfeeding, all my middle class friends pump and I was put under a lot of pressure to pump rather than give formula once a day by my GP and I just didn't want to. Equally, if you want to exclusively pump or combination feed or formula feed, those are all good options too.