Once is an adverb that simply means happening on one occasion. To remember the spelling, consider that the ‘c’ between ‘n’ and ‘e’ indicate you should take ‘care’ to slot it there. An example of its usage is this: Jesus’ sacrifice was offered once for all.
Ones, on the other hand, is a pronoun and it refers to people or things previously mentioned or easily identified. In spelling this, note that an ‘s’ follows ‘one’. An example of its usage is this: The beautiful ones are yet to be born 😅.