I plan on resigning as I am significantly underpaid for just 15 LPA(13 yrs exp). I'm a critical resource in my project. Will they retain me?

You need to have an offer to speak else things won't work

I read somewhere that it doesn't grant more than 40% as a counter offer. Is it true?

I am ep and having 19 yoe and my sal is 16.5

I resigned 40 days back
And they tried to retain with 40% hike.
They said that’s the max as per their policy.
Before giving retention offer they will try to see whether we are critical resource or not .
I would say one should have habit of switching in new environment.
Once you flow like a river from co to co then u will get the desired hikes.
It’s all demand and supply

Better to switch the company, if they retain Also they won’t give more than 40-50% which is very less for your experience.

If yes, then what kind of counter offer can I expect from them ?

Even if they give 40 percent majority they map it to Variable pay .. it will be scam

Yes 40 % with nearshore or onshore promise

You are grossly underpaid, this MFs don't deserve loyalty. They reward loyalty with 2 percent hikes. Pathetic mindset of these a**holes!

Go for it,you won't get money by sitting in your comfort zone. Even if they retain they won't give you 200% hike