There really is no hard and fast rule, so long as you aren't running out of memory that SpiceDB is using as cache. I'd personally recommend starting with a minimum of 2 vCPU and 2GiB of RAM allocated for an instance of SpiceDB and scaling up from there. Because SpiceDB is a latency focused application, aspects like having faster cores or being the only software running in a particular machine are going to come into effect more quickly than you'd think and can be more beneficial than adding more cores/ram.