Technically, there is no limitation to the number of interfaces that a system can have with the same link-local IP. https://tools.ietf.org/html/rfc4007#section-5
This is because the interface's name must appear in it's use in order to select it.
ie: ping6 fe80::dead:c0de won't work because it won't know what interface to use. You must use ping6 fe80::dead:c0d%em0.
That being said, FreeBSD 10.2 is perfectly happy with 3 interfaces configured as fe80::1, besides lo0 is also fe80::1.
fe80-1.png
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fe80-2.PNG
fe80-2.PNG_thumb