400w HPS for a 4x8x8 room ok for 20 plants? help me too on my list please

@topic
i got my seeds already Mandala Satori and Aurora Indica
my question is, is one 400w HPS enough for 20 plants? (not exactly 20 cuz i will be pulling some males out and transfer them somewhere else) i'm planning to veg them outdoors OR under a 400w MH
im afraid to store some seeds so im planting them all
im low on budget and i can only afford one set of HPS and one MH bulb

-intake and exhaust fans already installed
-a stand fan
-vinyl flooring, reflective sheets on walls installed
-still have to buy some pots - what size should i get?
-can i use worm castings+soiless mix+perlite - is this ok? these are the only available sterile stuff here, other than that they have some soil mixes but they have time release ferts already and they also have cocopeat and vermiculite
-already have a 50gallon reservoir with aerator pump, watering bucket, water sprayer/mister
-im gona make a diy carbon filter
-still searching for a timer

what else is missing for a poor man's indoor grow?

TIA guys!

400w is not gonna cover much of that room, it would a stretch to try to cover 4x4 with that, I think 3x3 will be your best bet. If you are serious and can afford it, 1000w wouldn't even make full use of the space. Since this is your first run at it, I might reccomend only doing half your seeds. That way if anything goes wrong you have a backup plan. The amount of plants you can do is based on how big you grow them. I've never done 12/12 from seed, but if you want to do that many at a time, might not be a bad way to go. Nothing wrong with just using a corner of that room you have, you'll appreciate having the extra work space.

A peat/perlite mix works well for me, the worm castings are good too. Maybe get some Dolomite lime for PH stability. I like a really well draining mix, so I go heavy on the perlite. Vermiculite and sand offer similar benefits. I personally like starting seeds in solo cups, but no reason why you can't go with 1 gallon out the gate. In a one gallon pot, I can get my girls decent size before absolutely needing a transplant. I have started using smart pots to replace my plastic 1 gallon pots, and it seems to all but eliminate root bound-ness.

Trying to visualize what you could put under a 400w, but maybe 6-9 1 gallon pots depending on how you grow your plants out. You could pretty much go with any size depending on how long you want to veg. You might do just as well ultimately with 2-3 large plants as 9 small plants stuffed in the space. For perspective, If you ultimately decide to go with something like 10-15gallon pots final size and a longer veg period(6 weeks or so), I wouldn't be able to get good light to any more than 2 plants under 1000w.

I grow fewer large plants, mainly because I need to stay under legal limits. I used to think this was ultimately easier because its just less individual plants to manage. I'm starting to see the benefits of smaller plants however. Namely pest treatments and such, which eventually you'll have to do. It is hard to treat the giants, as they are hard to move and too dense to reach all areas with sprays and such. A smaller plant like in a one gallon can be moved around easily and straight up dipped in a solution should you want to.

Stuff you will want:

Nutrients
-Tap water will probably work for you if you dont have excessively hard water.
-PH tester couldn't hurt, though I think a good balanced soil will work itself out. I've made good use out of a $12 ebay digital tester.
-Most even cheap timers are rated up to 15amp, so will work for 1 light up to 1k watts.
-For your DIY carbon filter, I've looked at the options. Not worth it IMO unless you can't buy one for whatever reason because even the cheapo quality ones work, and there are some pretty cheap options out there. I mean especially if DIY is gonna cost you anymore than say $50.

Check out my closet, im running a 400w hps magnetic ballast sunsystem and i have 15 plants in a smaller closet

400W 4-6 plants tops. You could grow all your plants under them but its gonna be great. If its not too late only grow a max of 6 under your lights. you could always take your clones the last week or two of your vegging cycle. that will be enough light for the clones to grow on the side. ones the roots show transfer your big plants to your flowering room if you have one and setup your clones to start vegging.