How do I know how much my baby is drinking if I’m breastfeeding?

If she is generally content and has wet nappies and gains weight then she is getting on just fine at the boob😊
You don’t need to know how many ounces she is having at the breast. Lactation consultants can do “weighted feeds” if necessary but usually that’s only if there is a concern like if baby isn’t gaining weight.
Breastfed babies are commonly always on your boobs - frequently and for long periods of time. I get that it’s gruelling compared to your friends who get to just do a big quick bottle every 3 hours. But it’s up to you how you prefer to feed your baby and what works for you 😊
After doing the research I made the personal choice to feed breastmilk exclusively from the breast, and I was lucky to have a years maternity leave so that it was possible

Oh and just to add about your pump output - pump output is not a measure of your supply. Babies are much more efficient at extracting milk from your breast than a plastic machine. Even with a good pump and the right sized parts many women don’t respond well to a pump. It’s common for breastfeeders to not get great output either because you’re pumping on top of feeding at the breast.
I’d usually get 40-60ml in a pump session - but I was able to exclusively breastfeed my bunch cluster feeder from the breast regardless