I'm a little late to the party, but it seems you've already done all the heavy lifting - good job! Now you can tell whoever said there was no drain plug to go sit quietly in the corner with the guys that say that you should never change the fluid in an "old" transmission.
You might have already figured this out, but I'll share some pointers for posterity.
The fluid you drain is typically 5 quarts, but will also vary depending on:
1. How the vehicle is tilted
2. How many components you disassemble (plug only vs. Dropping pan vs. Removing valvebody)
3. How long you allow it to drain
4. The temperature of the fluid when being measured
5. How much fluid the transmission had in the first place
Here are some videos:
1. How to measure quantity - https://youtu.be/QR1AV_jVxA8
2. How much to expect at draining - https://youtu.be/Nz5uhAEViUU
That being said, the codes you got are usually not merely "dirty fluid" codes, but if the fluid change has been working for you, it's all good.
I buy fluid from the dealer when servicing my vehicles for longevity. When troubleshooting issues, I prefer to spend a little less for fluid that might or might not be the issue - so I buy Valvoline Dex/Merc synthetic from either AutoZone (rewards) or Walmart (cost efficiency).
*I have found that this aftermarket fluid does not perform in any inferior way to OEM fluid.