Well, it varies by Pokemon and stats because they are not the same. But, the stat will double from each boost from the moves that you mentioned. For example, a Quagsire with 100 SP Defense uses Amnesia, the boost will double its SP Defense to 200 because Amnesia raises the stat by 2 stages. If you use Amnesia again, it will raise up to 300 SP Defense.
Basically, it will double your current stat. The highest those moves raise the stat is by 6 stages, which are 1.5, 2, 2.5, 3.5, and 4, meaning you can only use the moves 3 times.Note, I based this on the moves you mentioned, not moves like Harden, Howl, Tail Glow Etc. I hope this helps.