What are the valid users that need to exist on Mac OS X
There are 87 valid users. 83 from OS X 10.10.1 (Build 14B25) and 4 from Server 4.0.3 (Build 14S350).

You can see them from terminal with the command:

dscl . -list /Users


The list is as follows:

_amavisd
_appleevents
_appowner
_appserver
_ard
_assetcache
_astris
_atsserver
_avbdeviced
_calendar
_ces
_clamav
_coreaudiod
_coremediaiod
_cvmsroot
_cvs
_cyrus
_devdocs
_devicemgr
_displaypolicyd
_distnote
_dovecot
_dovenull
_dpaudio
_eppc
_ftp
_geod
_iconservices
_installassistant
_installer
_jabber
_kadmin_admin
_kadmin_changepw
_krb_anonymous
_krb_changepw
_krb_kadmin
_krb_kerberos
_krb_krbtgt
_krbfast
_krbtgt
_launchservicesd
_lda
_locationd
_lp
_mailman
_mcxalr
_mdnsresponder
_mysql
_netbios
_netstatistics
_networkd
_nsurlsessiond
_nsurlstoraged
_postfix
_postgres
_qtss
_sandbox
_screensaver
_scsd
_securityagent
_serialnumberd
_softwareupdate
_spotlight
_sshd
_svn
_taskgated
_teamsserver
_timezone
_tokend
_trustevaluationagent
_unknown
_update_sharing
_usbmuxd
_uucp
_warmd
_webauthserver
_windowserver
_www
_xcsbuildagent
_xcsbuildd
_xcscouch
_xcscredserver
_xcsd
com.apple.calendarserver
daemon
nobody
root


For completeness, my OS X (non-server app) Mac has 84 users.

It has an additional Guest user (FileVault 2 added that one) and is lacking these server apps.

_xcsbuildd
_xcscouch
_xcsd
com.apple.calendarserver