Breast and formula feeding...

Krys

First time mom here...

So I'm trying to start breastfeeding my 5 day old. I know my milk hasn't fully come in yet so I'm supplementing with formula. He'll latch and suck on each breast for about 15-20 minutes each, but then he will usually still have about 2oz of formula after... I'm not sure what to make of this... Does it mean he's not getting enough/any of my milk?

Any other moms or there having a similar issue? Any suggestions?