We support 30 developers on 10gb ram and 4 cores. That said, we are probably a year and a half behind version wise and only use it for its SCM capabilities.
We are running it on a 2vCore, 8GB RAM vm. Works good for our ~10 concurrent users on peak. The ~120CI builds per day are running on other hosts though.
quad core with 16 gigs of ram for a team of 4 people. Most likely 8gb of ram would be sufficient for us but the risk of decreasing our productivity is not worth the insignificant difference in price of 8GB vs 16GB VPS monthly fee.
For 5 users we use 18G/4vCPU. For 20 users we use 32G/8vCPU
I keep asking to lower the footprint but the gitlab is the very definition of feature creep. And that comes at the cost of optimisation.