TCL has a 65" 8k TV for about $2k, 75" for about 500 more. That's enough for 16 1080p streams (4x4) or 36 720p streams (6x6) and you won't be spending days banging your head against limitations on bandwidth, number of outputs, issues, or failure points, power outlets/chargers, image quality, and so on. On the other hand if it's for the fun of it I'm sure you can figure something like that out.

If "whatever is cheap" is what you are after a couple <$300 4k TVs will probably be better quality, cheaper, and easier to set up than 25 tablets.