You know there's an app for that, lol.
https://www.quikrete.com/calculator/main.asp
My area was 4'x12' and the calculator told me I needed 28 80lb bags to achieve a 4" slab. I purchased 32 to be safe and ended up using 26 bags. My space was actually only 11' long and not 12'. I had rounded up and then added another 10% so I wouldn't short myself on material.
The mixer is a cheap Harbor freight unit I borrowed from a coworker. It has a 3.5 cuft hopper and handles 2 - 80lb bags just fine. Add 3 quarts of water per bag and mixed for 4 minutes a batch. Starting at the back corner, I mixed 3 batches (6 bags) and then paused to level, screed and finish the area while I could still reach it.
There were some really good tips given earlier when I mentioned that I was doing the project so I will repeat them here. They were spot on and made a big difference knowing them beforehand.
1 - add the water to the mixer before the concrete.
2 - mix with the hopper as flat as possible without letting the mix spill out.
Other tips I learned in the process;
1 - Excess water washes the cement off the rocks. So does over mixing. An 80lb bag requires 3 qts of water. Adding as much as 1 extra qt reduces the strength by as much as 40%.
2 - Add your water, tilt the hopper straight up and bring to a stop to add your bags. Start the mixer back up and then tilt as flat as possible while it mixes. Adjust the angle as it mixes to achieve the most turnover and best agitation.
3 - Agitate the form to ensure you get a nice finish. A dry mixture will leave a crumbled and undesirable finish once you strip your forms. Easiest is to tap the form with a hammer to bring the smooth finish but others have used a vibrating sander up against the form. My father actually keeps a couple old palm sanders around and will seal them in a plastic bag and insert them right into the concrete on really important "aesthetic" pours.