Share your experience!
I do believe its an app issue, as other apps do show such number (email, gmail, messaging, facebook, etc) so if it work for some phones but not with Xperia, its something Whatsapp has to address.
That's not what I said, just because an app work with one phone it doesn't mean there won't be bugs on other phones like this
Have just tested this on my Z2 as i constantly use WhatsApp.......on the Xperia Home Launcher there is no 'message number' on the actual Icon but a symbol does appear in the notification bar (symbol, not a number).....i however use Nova Launcher as my Home Launcher which uses TeslaUnread Plugin for notification/unread numbers on Apps etc....works perfectly with WhatApp & everything else.
My suggestion to you would be to embrace what Android is designed for, design the phones operating system & contents/style to what you want.......if you prefer to stick with Xperia Home Launcher there are quite a few other Notification Apps on Google Play that you could try that should not change the actual Icon of WhatsApp, just a matter of trial & error.......
Just my opinion & i hope this helps.....sorry for jumping in @uliwooly :smileysmileywink: