What's the plural of "Confirmation of Entry"?

I've recently wondered about the plural of the expression "Confirmation of Entry". A "Confirmation of Entry" is a piece of paper that Cambridge Assessment English gives candidates who are going to take a test. This document shows the details of the candidates as well as the test date and schedule. So recently I read "Confirmation of Entries" on a website and it read really weird, so I thought that it would be more appropriate to write "Confirmations of Entry". However, someone else suggested that it would be more correct to make both plural ("Confirmations of Entries"), so now I'm really unsure as to which is the best option. I would appreciate some feedback. Thanks.

• Letter of recommendation (s)

• Letters of recommendation (pl)

• Letter of recommendations = A single letter with a list of recommendations

• Letters of recommendations = Two or more letters, each one listing several recommendations.

  Think of it like this:

We have one letter of confirmation (s)
We have two letters of confirmation (pl)
We have a confirmation of entry (s)
We have two confirmations of entry (pl)

There can only be one admission per exam, not a letter confirming several admissions for the same candidate entering for a single exam. Two confirmations refer to two individual candidates.

Confirmation is the main idea of the phrase, "of entry" describes the noun "confirmation" and does not stand alone.

Therefore, when creating the plural, the correct - as you said - is "confirmations of entry"

In title-case it represents the name of a process and is not yours to modify.

'Confirmation of Entry'

In lower-case it's your description of their process, so it's valid to add an 's'.

confirmations of entry

Another option is to append a pluralised noun such as "notifications":

The system sent out multiple 'Confirmation of Entry' notifications by mistake.