Depends how new you are to it.
I think in a 29 gallon you can get a good 12 cories, 6 of 2 species could be good. Personally I like Albino, Panda and Sterbai as cool looking ones. Albino are the cheapest and most common but I really like how they look and I find them to be one of the most active. Pandas stay smaller so you could get an extra couple of them compared to most other breeds and Sterbai I also like the orange/gold/yellow they have on their fin on a dark body.
You could maybe choose them based on your substrate, on darker then ones like Albino, Panda, Julii etc will stand out and on lighter like white sand darker ones like Sterbai, Bronze, Venezuelan black will stand out more.
I'd add them slowly, like one group of 6 and then another group of 6 a few weeks or months later. Give you time to look around at different types and see how you do initially.
A betta, 12 cories and i'd say a good school of 6-10 2 inch or under fish. You probably want a mid to upper level type, because the Corys give you action on the bottom and the Betta higher up the tank, you don't want a group like Neon Tetra who are mid to bottom would overcrowd the bottom level.
For example this would work:
6-7 Lemon/X-Ray/Cardinal Tetra or 8-10 like Cherry Barb or smaller style fish. Theres loads of types of tetra, barb or rasbora etc that would suit not ones larger than 2 inches ones or aggresive ones though like Tiger Barbs
6 Sterbai Cory
6 Panda Cory
Some people would probably say this is overstocking to have 20-ish fish in a 29 gallon but I think overstocking is when you are stressing fish, or dirtying your water too quick. These would be small fish and they're not fish who would stress each other. Although you would need to keep up with good 25-30% weekly water changes with a high stocking like this.
If you want a more relaxed time keeping the fish then maybe stick to 6/7 of the same Corys and only 6/7 smaller tetra types. Then you may only need a water change every 2nd week, maybe even 3rd week if you have plants.