If you have 3 Publisher sessions, then you must simply multiply by 3 the number of expected subscribers, and that will be the total number of streams in every session. Following the example, you can expect around 1200 streams per 16-core server. That is using Kurento as media server (OpenVidu CE, OpenVidu PRO). With mediasoup (OpenVidu ENTERPRISE, currently in beta and expected to be released very soon) you can expect up to 6 times that number (Home - OpenVidu Docs).
Of course bandwidth can be a bottle neck, that must always be taken into account.