Now, I'll preface this reply by restating that I don't know much about altcoins because >99% are outright scams as far as I am concerned, but I still think it is very common that people reuse the same seed phrase across multiple coins.

If you use a multi-coin mobile wallet as many people do, such as Coinomi, then you get one seed phrase across all your coins. If you use a hardware wallet, then you get one seed phrase across all your coins. I think it would only be if someone downloaded and used separate wallets for every coin (which I suspect very few people do) that someone would have different seed phrases for each coin.