While I agree with Ida that there's nothing wrong in supplementing with formula, don't be too quick to assume that you have to. Exclusive breastfeeding for the first six months is possible for most women but believing that you don't have enough milk is a very common worry that leads a lot of women to use formula even though they would prefer not to.
Not being able to produce enough milk is actually quite rare, according to this site https://www.breastfeedingbasics.com/articles/how-can-i-tell-if-my-baby-is-getting-enough-milk
There can be some health issues with the mother that lead to low milk supply, some of which can be corrected and there's also the possibility of the baby having a problem, such as tongue tie, which means they cannot suck milk from the breast efficiently. However, I think that lack of good advice and support, lack of confidence and lack of experience of seeing other people breastfeeding is probably a more likely reason that many women believe they don't have enough milk for their baby.
Of course it's definitely worth ruling out any medical issues with yourself or the baby if you are really worried. But if that turns out OK and you want to breastfeed exclusively, then I'd suggest getting some support and making an effort to meet other women who have breastfed exclusively. There are a lot of excellent resources online which should help to answer any questions you have. The site I linked above is good, as is kellymom.com and La leche league
When my baby was born, I worried a lot that I didn't have enough milk for him but for no real reason. Meeting with a lactation consultant, getting advice from other mothers who had breastfed and information from these websites all helped me to gain confidence and breastfeed successfully.
Here is some information about the issue and effects of women believing they don't produce enough milk when they do:
And here are some tips to understand if you do have low milk supply and some ways you may be able to increase it.
Some important things to note about how breastfeeding works are:
Breastfeeding is based on supply and demand so the best thing you can do to increase your supply is feed your baby as much as possible. If you have been using formula, it may take some time to increase your milk supply back to a level which will satisfy your baby so if you want to reduce the formula, you probably need to do so gradually.
Pumping is not a good indicator of how much milk your baby gets. Generally babies are much better at getting milk than a pump is so don't worry too much if you can't pump a lot of milk.
The best way to know if your baby is getting enough milk is if he gains weight well and has enough wet and dirty nappies (diapers). His urine should be pale in colour and he should have about 6 wet nappies per day. The number of expected dirty nappies is more variable after the first month or so but at the beginning you would expect 3-4 per day (this doesn't apply to this particular question but to anyone with a new baby).
Finally, just to reiterate my first point, there is nothing wrong with giving some formula for whatever reason, if you do have a real issue with milk supply or for some women and babies, breastfeeding just doesn't suit them. But if you would really like to breastfeed exclusively, don't give up without getting some good support and advice to help you establish if you have a real problem with milk supply or not and if you do, if there's anything that can be done about it.