There *is* a hardware sizing guide on MoS - I'm travelling right now but look again and let me know if you still can't find it. It's built on the kind of things that @Gianni Ceresa mentions - *actual* user concurrency, not just total registered users. Also need to consider are they running pre-canned reports or adhoc? Heavy trellis charts or 10 line data tables? Exporting to Excel 1M rows? All these kind of things have a huge impact on the server resource requirements.

You mention it's connecting to a DW and other sources - will you expect OBIEE to be federating data across the sources? If so that'll add to the resource requirements.

Is this a brand new system or port of an existing one? If the latter that can be useful starting point for server sizing (and user concurrency numbers)

500 *concurrent* users is a vastly different thing from 500 registered users. The rule of thumb I hear is 10% user concurrency, and from experience even that overstates it. Also bear in mind that concurrently logged in is not the same as concurrently executing work. And to get even more into it, they could be putting work on the Presentation Services by building reports, but nothing on the BI Server until they run them, etc.

In terms of HW resource, it's never a precise thing, because the workload is always so different. I've not seen anything in 12c that would make me change my general sizing estimates from those in 11g.

I'd look at the current spec of the 10g setup -- but also take the opportunity to evaluate *why* it's slow. Is it hardware resource, or is it poor OBIEE design? Because migrating a badly-designed 10g to a big box on 12c but still badly designed is still going to perform badly...