The last time Samsung launched a flagship Galaxy S model with 16GB of RAM was back in 2021. The Galaxy S21 Ultra, as well as 2020's Galaxy S20 Ultra, featured 16GB of RAM on the highest storage tier, which was 512GB on both models. Samsung reintroduced the 1TB storage variant with the S22 Ultra, but has stuck with a maximum on 12GB of RAM since then.
With next year's Ultra model, only the 256GB variant may feature 12GB of RAM. But since Samsung hasn't officially unveiled the Galaxy S25 series, you should take this and any other rumors with a pinch of salt so you don't have to face disappointment later on.
At this time the only thing we can consider a certainty is that the Galaxy S25 Ultra will have at least one variant with 16GB of RAM. The base Galaxy S25 model may also get a RAM upgrade, but from 8GB to 12GB. As for the Plus model, we expect all storage variants to feature 12GB of RAM.
