Hello, I intend to create a handmade 3d engine, now my question is for how many of episodes should I follow the handmade hero series? should I go through every episode so far because all of them contain some relevant information? thanks in advance.

Handmade hero is more about how to program then on making a specific engine. You'll get more early on then later. I'd suggest you join the discord server because part of the community has started to watch the episodes again and discussing them (they just started a few weeks ago, so you could easily join in).

thanks for the reply, in some other thread you said most people watch for 50 to 100 episodes and then they'll be able to figure stuff by themselves, so would you recommend watching till some episode near episode 100 is enough? also do the people in discord that you mentioned plan to watch all 600+ episodes of handmade hero?

You should watch until you think you're comfortable, and can do things on your own. I watched and coded along until the 100th episode (and I could have stop earlier in hindsight), and then watch casually the rest. There are bits here and there that are useful but after a while it's focused on solving problems for handmade hero specifically (sort of) so it's less useful.

I said 50 to 100, but people on the discord server seems to give a lower number than that (30 to 50). It's up to you really, and you can keep watching in a more relaxed way to see if there are other bits that interest you. There is also the episode guide that let you search for specific terms covered in the episodes.

Also note that Casey stopped HMH, it's not finished and will probably never be.

On discord they plan to watch all episodes. They discuss 2 or 3 episodes per week and I think they are around episode 10. I'm not following that so I can't give more details. I think it is probably the best way to follow as I believe they discuss things that aren't up to date anymore, for example you shouldn't use DirectSound for audio.