There isn't a one-size-fits-all answer, as it actually depends on the number and types of extensions. Furthermore, the type of game, especially if it's a modded version, also determines the amount of RAM you may need to play the game without any lag. Additionally, it depends on the number of active players on the server. More players equate to a greater amount of required RAM. The possibility of a RAM upgrade relies on the hosting provider, so it's advisable to inquire about this beforehand.