We can approach this analytically, by authority, or by observation.
Okay, to start with our components are at least straightforward; they're both countable nouns, and the plural of raspberry is raspberries and the plural of pi is pis. We'll be using them later.
It's formed from English nouns, so there are four possible ways to treat it.
We could just not think about it, and treat it opaquely, as if Raspberry Pi were a single word. That gives us a plural of Raspberry Pis.
We could treat it as a noun adjunct, where the first noun acts as an adjective. That gives us Raspberry Pis (c.f. coffee shops).
We could threat it as a headless noun, though that seems unlikely to be correct. Anyway, this would give us Raspberry Pis.
We could treat it like a compound starting with the head, which would give us Raspberries Pi, but we'd need some strong reason to favour the first word in this manner. We can rule this out.
Of the acceptable options, since it's a pun on "Raspberry Pie", we'd favour the noun-adjunct case, but they all have the same result.
Likewise, that gives us an etymological approach: It was named to deliberately be similar to "Raspberry Pie" so we should pluralise similarly to "Raspberry Pies". That gives us Raspberry Pis.
It was named by, and is a trademark of, The Raspberry Pi Foundation. They use the plural Raspberry Pis.
They've been called Raspberry Pis in a variety of places.
Hence, they're Raspberry Pis.