what if i go over my limit

Hi,

I just want to be able to stay within same monthly limit and that not to change. (and even RAM, just to be functional, so it can handle traffic)

Your instance type does not change unless you modify it. Also if you dont configure autoscaling but just 1 instance (or some static count) that's what would be running for as long as you keep it. If the load increases and the compute / RAM / storage is insufficient, then the instance would be unresponsive

I checked on-demand pricing, but got confused, it says "pay what you use", but that means "how much EC2 instance is running", it will be running for 2 years continuously..

in case of on demand instance the hourly price * tenure for which you keep it running is the final price, billed monthly. If you run it for only 10 days a month, then you get billed only for 10 days.

And then with Reserved Instances , I checked for 3 years to pay all upfront (Linux, Shared, US East (Ohio) , and I get some offers, like ( t2.micro $115 )

You get discounts since you making an upfront commitment. If your sure to use it for 3 years, you can reserve instance. You should also compare with Savings plans. They offer some flexibility compared to Reserved instances.

But do they depend on network traffic ? Or they just have some network bandwidth and that's it (no limit how much users I can have ? )

the instance type you choose has a network throughput limit and you can find that information here - https://aws.amazon.com/ec2/instance-types/. The total data transfer (IN & OUT) on these instances is independent of the throughtput limits. Download / upload speeds would be capped at the throughput speed

And if it goes over that limit , how I'm gonna be priced for more network usage ?

You are charged for data transfer in excess of limits. Refer Q -What does data transfer cost? in https://aws.amazon.com/lightsail/faq/

--Syd

EC2 network bandwidth varies by instance type and instance family, but there is no limit based on the number of users.
Also, the 1TB network transfer for AWS Lightsail that you are talking about is only charged for the amount over 1TB, not for the fact that you can only use up to 1TB.
https://aws.amazon.com/lightsail/pricing/?nc1=h_ls