Make sure your network can handle all the computers.
To add a bit more to this, I've been to quite a few LAN parties.
The most important thing is that you have a backup plan. Know from the get-go that thinks might now work out when linking all these PCs together. Every LAN party I've ever been to/hosted has had some sort of fallback just in case something fails. Typically, this means pulling out the N64 and Mario Kart or something like that, but it could be anything that you know people will have a fun time doing.
Secondly, hook up the computers one at a time. This makes it far easier to troubleshoot issues with the network. When troubleshooting or fixing issues, keep your guests entertained. Food is a great distraction.
Third, have an agenda, but keep it loose. I've been to LAN parties where people have sat around wasting time trying to decide what to do, which results in no fun. I've also been to LAN parties where there has been a strict order to things, which is equally no fun. Having LoL be the focus is a great idea, but make sure you have other options available just in case you get bored of LoL or run into issues. Free is best unless you know that other people have the game or you can provide copies of it.
Forth, provide additional things to do for the people not participating in the "main event". Simply having a movie on or something can provide that extra satisfaction and keep your guests happy.
Fifth, make sure you have enough food. Have everyone chip in a few bucks to buy food. Using pizza as an example, get enough pizza to use as a meal (around 2-4 pieces per person, this mainly depends on your friend group), then use all the rest of the money on drinks and snacks. Chips are the best option with snacks, cookies are a great second, especially if the cookies are homemade. Canned drinks work better than litters, but they are more expensive. Make sure you have 2 drinks per person per hour of the party. That will ensure you have enough to last the entire party. It's also important that you have a nice mix of drinks, meaning that you don't buy just cola or just mountain dew or whatever. Try polling your guests before the party and figuring out what drinks are most popular.
Sixth, check seating arrangements before you start. Try clearing out whatever room your plan on having everyone play in of all existing furniture. Folding tables and chairs are great.
Finally, make sure to have fun yourself. Organizing a party can be really stressful, so make sure you feel like you're not excluding yourself.
