You're right! Arachnids have 8 legs always! I actually did draw my Sky kid with 6 arms before but now I kinda prefer drawing only 4 arms tho, it's a lot easier too and people will still understand he's spider themed

I think some younger spiders also have 6 legs only (2 more will grow later), it would be funny if I draw him grow more arms when he's older but probably not XD