Difference between more batteries and boosting voltage
1.) Converter losses, but not too much. Good converters have 90-95% efficiency
2.) What is your load? If you have 9V 1A batteries as example, P= U*I = 9W.
At 18V you will be able to draw 500mA from 1 battery in this case. 2 batteries in series, which would give you 18V directly without boost, you can draw 500mA. Of course, 2 batteries also have a higher capacity, so you could power your project for longer before recharging batteries.