We’re going on seven months since the last time I did one of these. How appropriate.
As a word, seven comes from the Old English seofon, which in spite of the f was pronounced pretty much the same way. It comes from the Proto Germanic sebum and Proto Indo European septm, which also meant seven in spite of the fact that a b and a p are very obviously not a v or an f. No idea why they changed. They just did, and you can see it in words like September, not to mention the replacement of the s with an h in the prefix hepta- (you can thank Greek for that).
As for the numeral, the earliest version is the Brahmi version, which actually looks quite similar to our seven. In Hindu it looked a lot more like a 6 lying on its back, and then the Arabic version is like a v. When the numeral system migrated to Europe, a lot of different places used different versions of numbers, so it could look like a v or y a 7 depending on where you were. Which makes sense. It’s not like they were constantly connected to the internet back then. Places could go years or decades without interacting with each other. Who would have thought to standardize anything?
Tony Jebson’s page on the Origins of Old English