Performance implications of creating many, many sequences

Consider the following table:

CREATE TABLE invoice (
    account_id integer NOT NULL REFERENCES account,
    invoice_number integer NOT NULL,
    UNIQUE (account_id,invoice_number));

I would like to do the equivalent of making invoice_number a serial type, but on a per-account basis. Would it be a
reasonableapproach to create a separate sequence for each individual account? Are there performance implications I
shouldknow about, given that there will be hundreds of thousands of accounts? Is there another approach I should be
lookingat instead?