Do all applicants in an EB-5 case need separate biometrics appointment letters?

They should get separate letters. Because of COVID, the appointments have been slow to come.

If biometrics are required, USCIS should send an ASC appointment notice for each applicant (principal and dependents). In some cases, however, USCIS may be able to reuse biometrics that have been captured previously and are still present in USCIS systems. It is important to keep USCIS advised of your current residence address so you do not miss any appointments. You must file Form AR-11, change of address, within 10 days of moving to a new address.

They will get separate fingerprint appointments. Did they get separate receipt notices extending their evidence of green card status for 18 months? If not, follow up with USCIS.

Yes, generally all persons need biometrics except certain ages and they often forget to send notices for derivatives so you have to follow up repeatedly.

Everyone must have their own ASC letter in order to attend.

There is no harm trying but frankly, I doubt CIS will print someone without any biometrics notice.