How much traffic can I have?

my old hosting using ulimited bandwidth. but after my blog traffic more than their 3000 visitor in a day, they turned off my website.
whether the HostGator will not happen like that if I use shared hosting, sorry for my bad english

We do not have a limit to the number of visitors. However, there is a physical limit our network can handle.

If you bring so many visitors that it crashes our shared server, we will recommend you to a dedicated server.

I'm planning to buy a plan with you to use moodle for a local school, it will probably have around 300 users accessing at the same time, what would you recommend in this case?

Our shared hosting should easily handle that kind of traffic.

The answer really depends on the resources used by your script and users. Again, you are asking for very little now, but if you later outgrow the resources of our shared server policy, you will have to upgrade to a dedicated server.

how many users is the estimated limit (at the same time using sitebuilder) for tha Aluminum reseller plan?

I cannot estimate the limit for you, as this depends on the exact instant they click a link and what resources they are using.

However, 25 simultaneous connections should allow 300-400 users with minimal conflict.