How much and how often should a 6 month spit up ?

well my daughter is 6 months old and she spits up like half a bottle every feeding i have took her to the doctor 5 times they keep switching milk and telling me shes fine (i have to see a army doctor cause my hubby is in the army and im working to see a different doctor)but how much did ur baby or does ur baby weigh at 6 months im worried about her nutrition she weighs 13lbs 6oz is that to small well any advise or opinion please help thanks natasha