I suppose it will be debited monthly?
yea, a billing cycle usually lasts 1 month and so you'll be charged for the usage for that month at the end of the billing cycle, I'm not sure where the base-cost fits in since I've never been on such a plan but it'll either be at the start or end of the billing cycle, or maybe paired with the usage from the previous billing cycle
I see I will have a 8GB RAM and 8 vCPU per service
You can use up to 8GB ram and 8vCPU per service yes, although note that the amount you use will count towards your usage
Does it mean one account or each project?
One account can have multiple projects, and each project can have multiple services, see more info here: https://docs.railway.com/overview/the-basics
How many projects on an account? + How much bandwidth usage?
There isn't any such limits that I know of, as long as you're willing to pay for it I'm pretty sure you can go as far as you want