Mustama, I’d say 30, but that doesn’t all have to be in one person! I remember a colleague who said it was really hard to do game design as one person; you need to be creative, coming up with engaging ideas, but you also need to be rigorous in building the model that can implement it, and doing both together it a difficult tension. She argued you needed at least two people. Now, I’ve done it as one, but then I’ve been designing games for a long time ;). I think it’s more important that you understand the alignment and follow a good process (diverge/converge; experiment and prototype; etc) and have a diverse team with complementary skills and a shared vision, than the specific years of experience you have. Thanks for the query!

Best Regards from Finland