How to get unique number in Infopath field

Use following expression in text field value

normalize-space(

 concat(

 10/(string-length(userName())),

 substring-before(substring-after(today(), "-"), "-"),

 substring-after(substring-after(today(), "-"), "-"),

 substring-before(substring-after(now(), ":"), ":"),

 substring-after(substring-after(now(), ":"), ":")

 )

 )