There are no limitations on the number of characters in your pin number but short ones are less secure. A minimum of 8 characters is recommended.
e.g a 4 digit pin number would take a maximum of only 9999 guesses before the correct pin was found.
Do not use well known numbers such as your date of birth.
You can make your pin even more difficult to guess by adding a non numeric element. Long pressing (pressing and holding) the keys adds extra characters to your "pin number".
A pin number of 1234 could be made much more secure by adding an extra digit made up of a "Long press"
When setting your pin these alternate characters are indicated by an outlined star symbol instead of the usual filled star. When logging in, there is no difference and all stars will be filled.
It is not recommended to use too many of these alternate characters as logging in could become a slow and cumbersome process.