For a 14 gal You can go with any type of cory your want. It's your preference. If you get a smaller type of cory you can get more. If you get a bigger type, you have to get less (but no less than 3). I have previously owned bronze and albinos. My personal favorite are Julii's. Mine are only about 1.5 inches. I love their cream colored skin and stripes. Here is a pic of one of mine.
Errrr... I hate to burst bubbles, but those are most definately trilineatus, not julii. Julii are VERY VERY rarely captured and exported from Brazil, South America. C. Julii are found in the lower Amazon, where commercial fish collectors do not really operate. If they do find their way to pet stores, it's typically in Germany, but again this is very very very rare.
C. Julii will have very small and distinct spots, rather than the more linear pattern in your picture
Almost all pet stores label them as julii... but they are not :)
Maybe because it's easier to say "Julie-Eye" than "try-lin-knee-ah-tuss"?!
However, trilineatus (or "false julii") are super awesome!!
And... enough of my "rant".....
Anyways, I would recommend getting at least 6 of the same species. Most cories you'll find for sale are right around 2 inches when full grown. And make sure that they're compatible with the temperature you keep your betta at :)
Pick whatever species you like best!!
THEY'RE ALL GREAT!!