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

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

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

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

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