@yzgulec as people have rightly said above, no hard and fast rule. But when it comes to sizing compute for mixed workloads (or unknown workloads) I have used a 1:4 (pCPU:vCPU) contention ratio.
I also try and size 2GB of memory per vCPU (so 8GB RAM per pCPU), and with this 'rough guide' I find that my CPU and memory consumption scale nicely.
Obviously it gets difficult when someone has a workload that needs very low CPU but high memory (e.g. I've seen a 2-core VM with 64GB memory) and conversely high CPU and low relative memory (e.g. 8-core VM with 4GB RAM)... but these are typically exceptions.
Please share your decisions and build configs here, I'd be interested to see how it worked out in terms of compute:memory, costs, etc.