What can be stored in a QR Code?
Up to 7089 digits or 4296 characters, including punctuation marks and special characters, can be entered in one Code. In addition to numbers and characters, words and phrases (e.g. Internet addresses) can be encoded as well. As more data is added to the QR Code, the Code size increases and the Code structure becomes more complex.
Source: https://www.qr-code-generator.com/qr-code-marketing/qr-codes-basics/
Further detail into the type of data you can store:
QR Code Data Capacity
_______________________________________________________________
| Input | Max Characters | Possible Characters |
|______________|________________|_______________________________|
| Numeric | 7,089 | 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 |
|______________|________________|_______________________________|
| | | 0–9, A–Z (upper-case only), |
| Alphanumeric | 4,296 | space, $, %, *, +, -, ., /, : |
|______________|________________|_______________________________|
| Binary | 2,953 | ISO 8859-1 Characters |
|______________|________________|_______________________________|
| Kanji/kana | 1,817 | Shift JIS / X 0208 Characters |
|______________|________________|_______________________________|
Sources:
https://en.wikipedia.org/wiki/QR_code#Storage
http://qrcode.meetheed.com/question3.php
You can also find the official specification here:
https://www.iso.org/standard/62021.html