How do you make a good encounter for 7 level ones

I don't want my players dying quickly but I also want to give them a challenge.Problem is as I have been learning DND I realised that level 1 characters are literally glass cannons so I can't give them low health characters but I cant do too much damage either.

7 is a large party, so bear in mind that if they roll highly on initiative, you can expect them to destroy everything quite quickly, and if they roll low, they may take a few hits before they get to act.

With that in mind, aim for weak creatures in multitude, and spread them out - have some creatures need at least a turn of moving to get closer to the party, so that the first person to engage doesn't get dogpiled and smushed.

Give the party 2-3 minor healing potions and a healers kit, and explain to them what the healer's kit does. Don't pull the punches, but give the enemies a motive which doesn't warrant ensuring that the party dies - have them want to escape, or steal something, rather than keep hitting the party when they get back up. With 7 players, there should be a couple with healing abilities, so they should be quite self-sufficient. Make sure the fight is one which can be backed down from and it should be ok!

Give them a villain who isn't trying to kill them, or has an alternate goal in the fight. If the bad guy is trying to escape from prison, steal an artifact, or accomplish something else, then a loss on the party's part doesn't have to be a TPK 

If the enemy and their minions break into the vault to steal the One Ring, they'll absolutely try to neutralize any resistance so they can get out as soon as possible, but they're gone as soon as the players are out of their way and backup can administer medical aid so they can all talk afterwards about "how are we going to recover the One Ring!?"

Have an explanation handy for why the villain isn't really trying to kill all the players, and the pressure is off.