Imho If you ask them politely to get 8 kars, they will allow it. Anyway the price is 200czk per 10 minutes, helmets are for free, but its not so "clean" inside. Its better to take you own helmet or you can buy sort of racing-hood for 100czk there. No track-hire, just 200kc for 10minutes of racing. Its enough to buy 3x10 minutes... you hands etc. will hurt you as hell next day
About the connection between the center, its not so far away, but it quite long way to walk there. Almost impossbile imho
There is no metro station or train station, but there is a bus station "Vrážská" for bus number 165, 204 and 244. I have no idea from what place near the center this buses goes, I have been there only with a car, which is the best way to get there. if you will ask in some information center, they will help you for sure.
edit: LFS simulator was on the other side of city... Chaos is owner of that machine, so maybe ask him for more information - if there will be some action with Force Dynamics 301 nearby center of Prague.
If I can recommend something, it would be Kart Centrum Radotín. Its not quite far from the centre of prague and the track is really great. its the longest trackt in Czech Republic. Also there are great karts, but the price is a bit higher than on the other places here. How I sad, if you want to enjoy karting, go to Kart Centrum. There are some pics from meeting with LFS friends http://spangler.cz/photos/LFS_meeting/
edit: Race with 8 people is good, the first one can wait for other behind BTW there are 12 karts, so imho 8 racers is enough for racing.
edit2: Last pictures from my gallery are not from K.C.R.
a bit better than before. Still have the same feelings about the realism but it looks better. Anyway try to blur the background in same way as the trees before the background and aline the horizont, which has to be in horizontal ´position´.
I tryed to blur the background and imho its again a bit better. See the attachements