[mason55]

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It's not really the number of people in the hand per se but rather the pot size vs your equity.

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There are two different concepts at work here.

Let's work from the problem in the book since it demonstrates both.

The first issue is whether or not to call a bet with your draw. When it gets back to you you have to call 12 and there's 12*5 + 1*5 = 65. You have to pot odds to call the 12 because the flush comes in more often than 12:65 on the next card.

The second issue is whether or not to stick the rest of your chips in after you call the 12. In this hand it's correct because you're going to get enough opponents calling your raise that it will be profitable.

Say (somehow) the pot was 65 and you had to call 12 but there was only 1 person in the hand. In this case, you would call the 12 because you're getting the right odds, but you wouldn't want to put your last 8 in yet. You won't be making the correct amount of money off that 8 dollars when only one person calls it. However, if you miss on the turn and your opponent puts you all in, then you'd again be getting correct odds to call.

Another scenario is if you have A LOT of money left. Enough that people might actually fold if you put it all in. In that case it could be correct to push it all in in the hopes that you would take the pot right there. Be careful with this though because at low limits people don't like to fold.