See previous posts:
You have the disc decryption pw and the user pw (initially both set to 123456). Both support letters and a full keyboard is available but convenience may push using a lengthy/strong mixed character decrypt pw and numeric user pw as the numpad is shown by default with screen lock, for instance. Max length for LUKS is probably (as in, may change) 511 characters (as in 512-1, see cryptsetup --help). User pw may be limited by memory only (there is a good explanation here), but you may want to limit to less than hundred if you are hand typing on touch keyboard (not sure if there is memory for that, didn’t calculate, sorry). If my (bio)memory serves, you can set the rule for minimum length, requirement for mixed characters (or not), or even set that there is no pw (and to have no disc encryption, but that’s another matter).