How many millilitres of lip filler should you get?

I’d leave it to your practitioner’s artistic judgement. There’s a popular saying in aesthetic medicine, that why you go to see a great practitioner is because you are paying for ‘the skill, not the ml’. ie you want them to use their artistic eye to judge what is appropriate for you, bearing in mind the size of your lips, and what the rest of your face looks like, rather than just using up what they have in the syringe, regardless.