So I'm planning on playing a Thri-Kreen character soon in a friend's campaign, they'll have the Arcane Trickster Rogue subclass and I want to use disguise self so they can blend in with other races a little easier, but then after looking over the spell again, I noticed a part of it I pretty much forgot since before the Thri-Kreen this hadn't really been much of an issue yet (at least not for me).

You make yourself--including your clothing, armor, weapons, and other belongings on your person--look different until the spell ends or until you use your action to dismiss it. You can seem 1 foot shorter or taller and can appear thin, fat, or in between. You can't change your body type, so you must adopt a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.

This means they can't use this spell to look like a human for example since Thri-Kreen have 6 limbs (4 arms and 2 legs) and humans have 4 limbs, so how would that work? Are they just simply incapable of really using this spell (to look like another humanoid race), or is there a way to work around this?

To do what you want, you would basically have to use Alter Self, a 2nd level spell.

RAW yes. As a DM I never liked that language, and just went with as long as it's vaguely humanoid. I've allowed someone at my table to impersonate a Chitine, so I wouldn't go down hard on Thri-kreen using it to look Human.

Talk to your DM to see how they feel, but RAW the spell is effed for thri-kreen.

Quote from ZomblesKlein >>

RAW yes. As a DM I never liked that language, and just went with as long as it's vaguely humanoid. I've allowed someone at my table to impersonate a Chitine, so I wouldn't go down hard on Thri-kreen using it to look Human.

Talk to your DM to see how they feel, but RAW the spell is effed for thri-kreen.

 Thanks for the advice!

There is some advice indicating that all humanoids can use disguise self to look like any other humanoid, even Dragonborn and tieflings that have tails.

I would use that RAI, rather than a literal RAW

Quote from Mog_Dracov >>

There is some advice indicating that all humanoids can use disguise self to look like any other humanoid, even Dragonborn and tieflings that have tails.

I would use that RAI, rather than a literal RAW

 

 In the new book(s) Thri-Kreen are Monstrosities and not Humanoids, luckily the spell doesn't seem to care one way or the other about your "type" or the "type" you become.  

Quote from Neckerman >>

So I'm planning on playing a Thri-Kreen character soon in a friend's campaign, they'll have the Arcane Trickster Rogue subclass and I want to use disguise self so they can blend in with other races a little easier, but then after looking over the spell again, I noticed a part of it I pretty much forgot since before the Thri-Kreen this hadn't really been much of an issue yet (at least not for me).

You make yourself--including your clothing, armor, weapons, and other belongings on your person--look different until the spell ends or until you use your action to dismiss it. You can seem 1 foot shorter or taller and can appear thin, fat, or in between. You can't change your body type, so you must adopt a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.

This means they can't use this spell to look like a human for example since Thri-Kreen have 6 limbs (4 arms and 2 legs) and humans have 4 limbs, so how would that work? Are they just simply incapable of really using this spell (to look like another humanoid race), or is there a way to work around this?

 I'd say it wouldn't be able to cover up the additional limbs. BUT ... if a player used disguise self and then covered themselves in a cloak, they could get away with showing the face they chose via the spell.