Need a line number based on invoice number

I seem to be a little rusty on some of my SQL. I can easily get a row number grouped by an invoice number as follows:

SELECT
    ROW_NUMBER() OVER (PARTITION BY INVOICENUMBER ORDER BY LINEITEMNUMBER) AS INVOICELINENUMBER,
    *
FROM
    INVOICES
WHERE
    DATECLOSED IS NULL

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This handily gives me a unique line number starting with 1 for each invoice line item. What I can't seem to remember is how do I get another number that increments for each invoice number. So, say the first 2 rows in my data set have the same invoice number, they would both need to be assigned number 1. The next 3 rows are a different invoice number. They need to be number 2. Etc...