I have this in the potentialspawn.json

{
		"block": "minecraft:nether_brick",
		"mobs": [
			{
				"mob": "minecraft:blaze",
				"weight": 1,
				"groupcountmin": 1,
				"groupcountmax": 1
			}
		]
	}

And this in the spawn.json

{
		"mob": ["minecraft:blaze"],
		"maxlight": 7,
		"maxcount": {
			"amount": 1,
			"perchunk": true
		},
		"result": "allow"
	}

Even with amount set to 1 tons of blazes spawn.

This doesn't happen with Thermal's Blizz even if amount is set to an higher value
potentialspawn.json

{
		"block": "minecraft:snow",
		"mobs": [
			{
				"mob": "thermalfoundation:blizz",
				"weight": 1,
				"groupcountmin": 1,
				"groupcountmax": 1
			}
		]
	}

spawn.json

{
		"mob": ["thermalfoundation:blizz"],
		"maxlight": 7,
		"result": "allow",
		"maxcount": {
			"amount": 8,
			"perchunk": true
		}
	}

Full potentialspawn.json and spawn.json: https://gist.github.com/Insane96/d036d0c12427473ddb9d64c94a5f58af

"allow" is the standard. You need to 'deny' spawns if there are too many of a mob. If you don't have any rules with 'deny' in then spawns will always work

How can I do it? E.g. if I want to make blaze rarity like skeletons?
Something like:

{
		"mob": ["minecraft:blaze"],
		"mincount": {
			"amount": 1,
			"perchunk": true
		},
		"result": "deny"
	}

?

perhaps. Try it out :-)

But why it doesn't happen for Thermal's Elementals?

No idea. Perhaps blizzes have their own built-in restrictions?

Uhm probably. Btw with mincount and deny now it works good. Thanks

Ehm almost working.
Blazes ignore light level, they spawn even when the area is lit

{
	"mob": ["minecraft:blaze"],
	"maxlight": 7,
	"maxcount": {
		"amount": 8,
		"perchunk": true
	},
	"result": "allow"
},
{
	"mob": ["minecraft:blaze"],
	"minlight": 8,
	"mincount": {
		"amount": 8,
		"perchunk": true
	},
	"result": "deny"
}

Use "default" instead of "allow"

Still spawning ignoring light level

Remove your maxlight/minlight tests

Nope

Turn on /ctrldebug mode and check your output (log) to see which rules are succeeding. Maybe that gives more info

Also give me your full current json's (everything)

Log after I've enabled degug mode: https://gist.github.com/Insane96/0ada6248ef054dff52f5cdf3e0742476

potentialspawn.json and spawn.json: https://gist.github.com/Insane96/d620e5af1b2969f037e0e82ad1e97b9e