N/A

The hobby plan is $5 with $5 of usage included + additional usage beyond the included $5.

https://docs.railway.com/reference/pricing/plans#included-usage

Examples:

If your resource usage is $3, your total bill for the cycle will be $5. You are only charged the subscription fee because your resource usage is below $5 and therefore included in your subscription

If your resource usage is $7, your total bill for the cycle will be $7 ($5 subscription fee + $2 of usage), because your resource usage exceeds the included resource usage

so when i have like 5 projects each having 10 services running how much credits is it gonna cost per month

This completely depends on how much each of your service is consuming. Railway is usage based and doesn't charge for the amount of services, meaning that they only charge for the vCPU, RAM, Egress and Disk (Volume) used by your services.

For more info, please take a look at the pricing page https://railway.com/pricing