[RPG] Can players handle Deadly encounters if they don’t engage in any other danger

All D20, but DnD 5 especially, are designed and balanced assuming a 4 or 5 player party. And you can kinda tell, when the rules start asking you to apply multipliers to bigger parties, rather than giving any concrete guidelines. Assuming distributed player competence, a 6 person party isn't simply 20% stronger than a 5 player party. It's much higher. There's an entire additional PC, with their own suite of abilities, magic items, and most importantly, actions. Never underestimate the power of having more standard actions than the other guys (unless you're a pack of CR1/2 minions going up against a bunch of level 5 adventures, then you're screwed either way).

In short, designing challenging encounters for big parties is one of the more substantial challenges a GM may have to face. There seems to be a razor thin design space between "no actual danger" and "guaranteed TPK" when planning for large parties. My own personal strategy is "5 players per party max unless you have a super good reason that a sixth needs to be in this particular game, and never ever ever ever more than that (and preferably not 6 for long)," but that probably won't help you, specifically.

First of all, stop giving them one big thing to focus on. There are a couple iconic encounters that tend to necessitate one big monster against a party of intrepid heroes, like dragon slaying. The problem here with big parties is 1 creature generally can only attack one thing at a time, so even if the beastie is downing 1 PC a round, the rest of the party can pop cool downs and beat the timer. If you design the combat space so the beastie can use it's AOE abilites to good effect, you often find yourself looking at a TPK. Quantity may be a quality all it's own, but it isn't everything. Basically, when the party is that big, the single monster encounter HAS to be able to one round KO any given PC, or it's not a threat. And while it's killing one PC a round, the remaining party members HAVE to have a DPR high enough to whittle the beastie down before it kills them all, or they all just die. SO! Anytime you're tempted to let 7 or 8 dungeon crawlers dogpile one big boss monster, resist the urge. Instead of fighting one wyrm, why not a mated pair of adults, maybe with a wyrmling thrown in to harass the squishies? (Actually, this particular piece of design advice sort of holds true for any size party if there's more than one healer to keep the front liners standing)

Secondly (related to the first), if your party outnumbers the monsters, they'll probably win unless each monster is SIGNIFICANTLY stronger than the PC. I can't give concrete CR equivalencies because it's different at different power bands, but 7 lvl 5 PCs who know which end of a longsword is pointy should mop the floor with 5 CR7-8 monsters. The power of two players worth of extra actions is too substantial to ignore. Design the encounter using the guidelines in the DMG, then add a few support casters or bowmen(or bowwomen, or bowgoblins, or bowwhatevers) at a little under CR to bring the numbers within 1 of the party.

Finally, if you're going to do mob (mob in this context being a large angry group, not a MMO enemy, nor a crime family) encounters, consider looking at the minion style monsters from 4th edition. They had decent defenses and attack bonuses, but 1 HP and very low damage. Custom brew up something mob-able, maybe give them advantage for being adjacent to allies, with a decent attack bonus and a beefy AC, but a damage range of a d3 and very low health. And remember, mob fights aren't typically meant to be challenging in and of themselves, they should be hard enough to drain some resources while letting the players feel like badasses for steamrolling through a pile of enemies. Remember, damage spread to 7 players hurts a party much worse than that damage stacked on 1 or 2 frontliners.

I'm currently running a West Marches style game. I realised before I even began that, if I were to use the standard resting times, I would either have to run an excessive number of random encounters, or have each and every one steam-rolled by a fully-rested party.

My solution was quite simple. I changed short rests to 8 hours (and renamed them to simply 'rests'), and completely removed long rests, instead saying that, at the start of every new session, the party is fully rested. Because, in the West Marches format, every session is a new one-shot (of sorts), and every party starts and ends in town, it seems natural that the party should start each session rested (having just been staying in town).

As a result, each session contains a reasonable number of encounters (including traps, environmental hazards, etc.), such that the 'adventuring day' is in fact split over a week (or so) of in-game time. This keeps the number of random encounters each (in-game) day feeling natural (one or two), while still taxing the party's resources towards the end of the session.

It seems to be working well, although I foresee that resources could be stretched rather too thin if the session were allowed to run on longer (than about 6 hours), or an adventuring day were allowed to run across multiple sessions.

In theory, it would be possible to do this in any campaign, as long as the players were willing to accept the slight implausibility of being fully-rested at the start of every session, regardless of circumstances. The key is recognising that it is the long rest, not the short rest, which has the greatest potential to imbalance the game. Because recovering hit points in short rests is tied to hit dice, allowing your party as many short rests as they like is unlikely to radically affect balance.

To summarise: The 'adventuring day' need not be an actual in-game day. It can instead be recast as the time elapsing in one session.