What I mean by meshes here are those nets that make a lot of tiny holes for lights to come in. Do I need light portals equal to the amount of the holes or just a big one? I tried to use a big portal that cover the entire holes for each window object but it still quite noisy inside. I use 20 samples for rendering.
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You can use a big one!
Two in your case; left wall, end wall.
Remember to align the portal direction (it works one way). And it doesn’t work with sunlight, only environmental maps.
And in the final render you can crank up the samples.
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Thank you for your feedback. 
Yes, I do think 20 samples are just too low. I was wondering how Mikey got away with it on his four-holes example and I thought there must be another factors that affecting this. But now, knowing that the light on my scene itself quite low in amount, I just need to crank up the samples. Cheers!