Hi PawelZ,
I was also looking through a similar concern and came across the datasheet for MX80 platform. It says 4GB of NAND flash for storage.
https://www.juniper.net/us/en/products-services/routing/mx-series/mx80/
https://www.juniper.net/assets/us/en/local/pdf/datasheets/1000597-en.pdf
You can check this detail by using "show chassis hardware detail" on your MX80. However, below is one such snippet from my MX80:
nex@raptor> show chassis hardware detail
Hardware inventory:
Item Version Part number Serial number Description
Chassis H1468 MX80-48T
Midplane REV 11 711-031603 CACT9711 MX80-48T
PEM 0 Rev 05 740-028288 XL13121 AC Power Entry Module
Routing Engine BUILTIN BUILTIN Routing Engine
da0 3820 MB QYR4000F4 Nand Flash 0
da1 3820 MB QYR4000F4 Nand Flash 1
So I suppose if you have 4GB already, you can have one more. A total of 8GB.
Hit kudos if that helped, mark "accept as a solution" so that other enthusiasts are directed here directly without them having to repost!
//Nex