Almost anything you can turn into an image and want in order — speeches, lists, facts, numbers, names, whole decks of cards.

Classic uses:

• Speeches / talks — one locus per point, walked as you speak (the original Roman use).
• Study material — the bones of a heart, the causes of a war, vocabulary, formulas.
• Everyday lists — shopping, to-dos, errands.
• Numbers & cards — paired with an image system (see below), this is how memory athletes hold hundreds of digits or a shuffled deck.

If it can become a picture and benefits from order, it can live in a palace.

Hundreds to thousands, organised across many catalogued palaces.

Competitive disciplines demand huge capacity — memorising thousands of digits or many decks in an hour — so athletes build and maintain large libraries of palaces, each with dozens of loci, all rehearsed and labelled. Crucially they built up to it from small palaces; the number is the product of years of practice, not a starting requirement. For everyday use, a handful of solid palaces is plenty.