Does it depend on 1 month or on usage of 1 month

Railway's pricing is a mix of a flat upfront payment and usage based payment

Pro plan has a flat rate of 20 dollars a month and then you're also charged for usage after that. If you were to upgrade to Pro plan you'd be charged 20 dollars (per person in your team) and then you'll be charged more money as you use the pro plan. If you remove all deployments in a way where you won't incurr usage then you won't be charged more money but at that point you would've already paid the upfront 20 dollars and any usage incurred until that point in time.

So to answer your question, the pricing is dependant on both. Pro plan is 20 dollar flat rate + usage.

I stop all deployed projects and don't use railway. Will my days still count. I mean will my membership still ends after end of month

Your membership typically auto-renews but you can cancel it yourself