My 6 month old breast fed baby only drinks 5 ounces per bottle. She bottle feeds twice while I work for 8 hours. She is 18 pounds. She was born 8.5 oz?

By weight is fine. She is likely getting much more when nursing directly from the breast. While you might expect her take more from the bottle at this size, she may prefer the breast and baby foods (presuming she's being fed some). As long as she's wetting aces diapers in that 8 hrs, is happy after feeds , don't worry. Offer a cup with water, too- it's time to start learning.
Sounds OK. We expect babies to double birthweight by 4 -5 months and triple it by a year. She is about on track with respect to her weight. The 5 ounce bottles are a reasonable size. Presumably she is breastfeeding or taking solids at other times of the day. In any case, her weight gain suggests that she is getting enough calories.
Underfed but Growing. A 6 month girl at 18 lbs is around the 90th%ile for weight so the concern is about her being overweight. However at this age she should be getting approximately 7-8oz bottles 3-4x a day. It is unusual that she is gaining weight but might be underfed. Have you been introducing solid foods? You can do that now one new item at a time. Continue breastfeeding, it provides valuable immune protection.