Caveat: this is NOT a judgement on formula feeding or topups, but I did a lot of reading about breastfeeding and wanted to share some stuff that might not be common knowledge. So when it comes to breastfeeding/formula-feeding you can make informed decisions that suit your personal circumstances and baby. I also recommend the Breast/Bottlefeeding forum on here, there are some people on there who know lots about this stuff.
You don't actually ever run out of milk. Before a feed, your boobs will feel full and heavy, because milk accumulates during the time you're not feeding. Then when the baby drinks they're 'emptying' your boob, usually drinking with quite big gulps as they drink the foremilk. As your baby feeds your body actually continues to produce milk. After a while the milk slows down to more of a trickle, but this is the good milk. It's the 'hindmilk' that is more fatty and will fill the baby up more than the watery 'foremilk'.
As long as the baby is suckling, your boobs are stimulated to make more milk. It also tells your body about your baby's needs. So if after a feed your baby seems hungry, it doesn't mean you don't have enough milk, have 'run out' or your milk is not filling them up, it just means you should put them back to the breast so they can have more and stimulate your body into making more (as it also has a knock-on effect to the next day). If at the end of a feed you give formula, you're basically taking away the stimulus that tells your body how much milk it needs to make for your baby, which results in lower supply (and can give the impression that your baby needs formula).
I just downloaded an app called iBaby Feed Timer. It's quite helpful to gather some data about your baby's feeding habits (whether they be breastfeeding, expressed milk, formula or mixed feeding) and helps you remember which breast to offer! It also helps you keep track of how long your baby has been on one breast and you can set a minimum time for it before it tells you to switch, which is helpful to make sure your baby is getting to the richer hindmilk. It's definitely showing me some stuff about Z's feeding that I didn't realise. It also has pretty graphs and statistics 