update records to have occurance number

I want to update a table to have the value of the occurrence number. For
instance, I have the below table. I want to update the number column to
increment the count of last name occurrences, so that it looks like this:

first last 1
second last 2
third last 3
first other 1
next other 2

Here's my simple table:

create table person ( fname text, lname text, number integer);

insert into person (fname, lname) values ('first', 'last');
insert into person (fname, lname) values ('second', 'last');
insert into person (fname, lname) values ('third', 'last');

insert into person (fname, lname) values ('first', 'other');
insert into person (fname, lname) values ('next', 'other');

How would I issue an update statement to update the number column?

thanks!