The low-tier Legendaries (i.e. Latias/Latios) are usually only 1 per game (I think), and usually have 600 BST.
The cover Legendaries (i.e. Groudon and Kyogre, and Mewtwo, Dialga, and Palkia, are in this category as well) are also only 1 per game, and are called Special or Restricted Pokémon, with the latter being a term used in VGC. Those are usually not allowed in VGC, but are allowed in some years, i.e. 2016, 2019, and 2022 right now, and players can have a maximum of two of them on their team during those years.
IIRC, Mythicals (i.e. Arceus, and the other Mystery Gift ones) were never allowed in VGC.
There are three separate designations in the coding of the games starting in Sun and Moon:
-Sub Legendaries: For the most part they are allowed in most formats with no restrictions outside of a handful of special tournaments(Note that while they are not actually Legendary lore-wise, Ultra Beasts are flagged as sub-legendaries in the coding of the games):
Articuno. Zapdos, Moltres, Raikou, Entei. Suicune, Regirock, Regice, Registeel, Latias, Latios, Uxie, Mesprit, Azelf, Heatran, Regigigas, Cresselia, Cobalion, Terrakion, Virizion, Tornadus, Thundurus, Landorus, Type: Null, Silvally, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, Nihilego, Buzzwole, Pherosmosa, Xurkitree, Celesteela, Kartana, Guzzlord, Poipole, Naganadel, Stakataka, Blacephalon, Kubfu, Urshifu, Regiekeki, Regidrago, Glastrier, Spectrier, and Enamorus
-Restricted Legendaries: Banned from most battle facilities and often limited to one or two per team in official tournaments if they are allowed at all:
Mewtwo, Lugia, Ho-oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas, Yveltal, Zygarde, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Zacian, Zamazenta, Eternatus, Calyrex, and most likely both Koraidon and Miraidon
-Mythicals: Banned from most battle facilities, and are never allowed in tournaments outside of very extreme circumstances(so far only in the very final Gen VII wifi tournament where any species caught in any game could be used in a free-for-all as a big grand finale to the 3DS era of the series):
Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, Hoopa, Volcanion, Magearna, Marshadow, Zeraora, Meltan, Melmetal, and Zarude