Let's say I have a standard pre-flop open raising chart which I follow religiously. But, now I'm facing a table with multiple limpers who often call your raise, what's the most optimal adjustment when it's my turn to act if I want to raise?

There are two options

Raise with the same range and raise more for each limper
Raise with a tighter range and raise more for each limper

What I think is that if these limpers are limping with ATC, I can clearly raise with the same range. However, we know that's probably not the case. But, even knowing they don't limp with ATC, do I need to adjust my range? Or, do I need a totally different strategy, like limping with them at in position with certain hands?

For the sake of a strictly theoretical discussion, let's not take into account the stakes level.

I would propose a third option, which is: Throw the chart in the bin.

The problem with using charts is, you dont actually know why to do certain things (since following a chart doesnt require to think about it). Hence, when you are confronted with situations that your chart isnt made for, you are lost.

In your example, I can give you the same advice I always give when someone asks a broad question: It depends.

Is there some heavy maniac behind you? Limp along your strong hands and let him raise for you the first time.
Are the guys limp-callers or rather limp-folders?
Are you MP2, CO, BTN? If not BTN what kind of players are the guys that would play IP vs you?
....

Now all of this is probably what you werent asking. And what you want to know is this:

Raise a tighter range, raise it bigger, limp along with plenty of hands that have good odds for strong hands, like pps and scs.

I agree with ronny. However, it is extremely useful to have somewhat standardized ranges, and charts is a good way of keeping track of what they are. After all we often find ourself in situations where we have little or no information about the other players. This is especially true for microstakes and zoom.

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Look at your opposition and look at your hand. A hand like AA works well vs 2 players. So figure out a sizing that will make 1 or max 2 ppl call. A hand like 44 you might be happy to limp behind with. Against bad players you can play straightforward like that because they still look at their hand so much and have problems visualizing what you might have.

Standardising ranges is smt bad players do to learn to cope with themselves not 3betting 40% of their range. And it's smt that weak TAGs do because they try to emulate someone else's playstyle.

Just try to suck as much money as possible from a player.

Vs one bad limper: I raise to 4 big blinds with a range that plays well heads up depending on my position. From early position, I like a range like this: 99+, ATs+, AJo+, KJs+, KQo, QJs. From the button, I like a wider range like this: 66+, A2s+, A8o+, K9s+, KJo+, Q9s+, QJo, J9s+, JTo, T9s, 98s.

Vs two bad limpers: I raise to 5 big blinds with a range that plays well multiway depending on my position. From middle position, I like a range like this: TT+, ATs+, AQo+, KJs+, KQo, QTs+, J9s+, T9s. From the button, I like a wider range like this: 77+, A2s-A5s, ATs+, AJo+, K9s+, KQo, Q9s+, J9s+, T9s, 98s, 87s.

Vs three bad limpers: Online, I just click the "pot" button. Live 1/2, I make it 15 bucks with a range that plays well multiway depending on my position. From middle position, I like a range like this: JJ+, ATs+, AQo+, KJs+, QJs, JTs. From the button, I like a wider range like this: 99+, A2s-A5s, ATs+, AJo+, KTs+, KQo, QTs, JTs, T9s.

These ranges are significantly different from my opening ranges from the respective positions. For example, I'll open raise 76s on the button first in, but that hand isn't in any of my raising ranges vs limpers.

What is more important is your post flop play. You are investing more on pre flop action than you are winning post flop. Rather than worrying about range and amounts, think in terms of risk and reward. You are not being rewarded properly for the risk you are creating by something in your betting and/or playing style.

This is one of the weaknesses of a "GTO bot" that only plays well against opponents that play just like itself. In other words, opponents that never open-limp, and only bet specific fractions of the pot.

The simple answer to your question is just to adjust your PF chart based on how well the hands play in multi-way pots. Raise a larger amount with hands that don't do well. A hand like a medium pocket pair can be limped along. Although in theory it has the equity to raise PF against the junk your opponents are playing, in practice it will have a hard time realizing that equity when it misses the flop.