Engaging More Enemies than You Have Allies Present and the Basic Math

Every champion has 3 basic abilities, one ultimate ability, and every player has 2 summoner spells.

The strength of a team fight in which all champions are of relatively equal power corresponding to their counterparts (Bot=Bot, Supp=Supp.Mid=Mid, Jg=Jg, Top=Top) is generally equal to the number of abilities and spells they are able to apply to the fight.

If each champion and player has full access to all of their abilities and spells then the strength of a given champion is then translated into the number 6 for the sake of argument.

So, in that scenario, going in as a lone champion or two or three against an entire enemy team is you pitting your 6-18 power against their 30 power.

In other words, they will have anywhere between 5x and almost twice the power you have in a fight in which the enenies outnumber you, making it so that you will likely lose with the exception of having already been fed in the first place, but even then mistakes are made and matches are lost because players overestimate their power against the power of their opponents.