My 5.5 month old is pretty much the same - sometimes takes 2 oz and other times it’s 4 oz every 3 ish hours. Shes on the smaller side, and I try to remember not to force her. Shes hydrated, happy, and reaching milestones!
As long as the baby is growing and having wet diapers (which signals proper hydration), nothing is wrong. I have twins and one drinks 4-5oz and the other drinks 2-3 oz, but the ine that drinks less is bigger than the one that drinks lore. They both are healthy and growing.
I've always been told 24-32oz per day is the target range for milk. If babe is growing in a way your dr is happy with I'm sure all is going well. If you're unsure about babe's eating best to consult with your dr