It seems like I get the same number of spiders by just having half ranger half druid. If I stick to leveling those two, do you think the companion and the wild shape will be useful? Or are these really only good if you single class?
Yup, it'll be the same number of spiders.
For the Beast Master companion, level 5 is a pretty important milestone - it gives your companion an additional bonus to AC, damage, and attack rolls equal to your proficiency bonus. This is a bonus that can increase even if your multiclass, so I'd suggest at least level 5 on Ranger. However, the companion will continue to get stronger and get additional abilities all the way up to level 11. The companion upgrades are at levels 5, 7, 8, and 11.
For the druid Wildshape, your spider form will get an HP increase every 2 levels. At levels 4 and 8, it will also get a damage die increase. At levels 5 and 10, you get additional attacks while Wildshaped.
With a 6/6 build, your Beast Master companion Spider will have 24 HP, 19 AC, and one attack per turn. Your Spider Wildshape will have 39 HP, 14 AC, and two attacks per turn. However, outside of Wildshape, your human form will have at least 69 HP (+ 12*CON modifier) and be able to use all of your armor and weapon magical properties along with having two attacks per turn. In my opinion, one of the two spiders (either companion Spider or Wildshape Spider) can be made effective, but probably not both.