Adverbs derived with the sti suffix from stems of cardinal numerals express how many times something occurs: yhdesti, kahdesti, kolmesti (∼ kolmasti), neljästi etc. mean once, twice, three times, four times, etc. Instead of yhdesti, the word kerran (genitive of kerta) can be used and is much more common. As a less common synonym for kolmesti, the word kolmasti is also used.
An alternative is to use the noun kerta and a cardinal numeral: yhden kerran (or just kerran), kaksi kertaa, kolme kertaa, neljä kertaa etc. These are more common for larger and more complicated numbers, e.g. viisitoista kertaa (rather than viidestitoista).