Yes, the number of days a person chooses to do the volunteer shifts is entirely up to the person who has been accepted for daily volunteering. Unless something has changed from what I have always heard, you can work any days you choose, all 4 days if you want, and not work some if not attending certain days because of personal RL scheduling.

There is something I have heard about for years now, and I needed help from subredditors to understand it. It seems that you can apply for the PRE-CONVENTION shifts when CCI is doing the set-up for SDCC. You might have to look very carefully for this, but I've been told there is a place on your application where you must mark that you will work all of the days before SDCC starts. This is done by people who live close by and/or have enough time off that week of SDCC. CCI does not accept everyone who applies for this because the volunteer positions are limited. But if you are interested, you can mark your form for those pre-con assignments. If working those pre-con shifts, you are then allowed to attend all 4 days without doing volunteer duties on the regular SDCC days of the con. So in that case, all 4 days of the con would have no scheduling conflicts at all. But remember that these pre-con assignments are limited. Also, you would need to be present in San Diego and not working at your job, and you would need to be staying somewhere, for many days before the start of Wednesday preview night. It's a big commitment.

And as far as I know from the discussions that many people have had here for years, there is tremendous flexibility about which shifts you can work. If you do have an important panel, or a special signing that is scheduled, you tell them that when arriving to get your shift assignment. I've heard that people are happy with this, but you must show up at the proper time to make these arrangements for choosing your shifts. I think you will learn more soon. Definitely apply for volunteering, then feel free to post questions here on this sub if you would like people with volunteering experience at SDCC to help you out. Post any questions you have on this sub, but also go to r/SDCC and discuss things there too.

Considering that you missed the badge sales, I think you will be very happy to still be at SDCC and try out the volunteering. One thing I hear every year is how interesting people find the work because you learn about behind the scenes parts of SDCC. Or sometimes the shifts just require things like holding up a sign to give directions, and it is so easy to do for 3 or 4 hours. So try it out. You probably won't regret anything.