You have to decide to either use odds or percentages. For example:
In the pot: 10BB
Vilian bets: 5BB
Another one raises to 15BB
In the pot now there is 30BB.
You have to call 15BB to win a total of 45BB (what is now plus what you add)
Your break even percentages is 15/45 which is 1/3 = 33%
You need about 17 outs (17*2=34) to call if you are behind. But if this is your all-in, so you are sure you don't put anything more on turn and river, 9 outs is enough (9*4=36).
Of course you also have to consider your implied odds, so depending on their stacks, 13 outs might be enough to call. You also have to consider what initial betting player might do. But that is another story.
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