No - there's always an equal number of contract outstanding on both sides of the bet. A contract is a promise from the person who sold "no" to pay the person who bought "yes" a dollar if the outcome happens. These contracts can trade from anywhere between 1 cent to 99 cents corresponding to a 1% chance to a 99% chance that you would die*. The odds the market reports is just whatever price the last contract traded at (or alternatively whatever price sits between the current open offers to buy/sell contracts. In liquid markets these tend to be the same).
> If nothing changes about the market and I'm still alive at the end of the day, everyone who holds a "No" share splits the $500,000 pot, correct? There are 4,500 of them, so they each get $111.11 per share.
They each get $1 per share. Their profit is $1 minus how much they paid for the share. It's not (meaningfully) a shared pot which is divided up, it's a fixed amount per share.
> They decide they want to dump $50,000 in on the "Yes" side. That's not going to buy them 500 shares, because they would need someone willing to sell 500 shares at the current price.
Ignoring the numbers at this point - you're generally right that they need to find someone willing to sell them the contracts. The existence of a large number of outstanding contracts doesn't guarantee this - they might be held by someone who is holding them to minimize the payout a hitman could get for killing you for instance.
The most direct guarantee is the order book The order book is the collection of open offers "I'm willing to sell X yes-contracts at Y price" that the market has for potential purchasers. The hitman can look at this and snatch up all of these simultaneously (up to some race conditions in the market - we can mostly pretend those don't exist but they do introduce some risk on the hitmans side). This can be thought of as the size of the currently available bounty.
There's a chance the market will continually over-price these yes contracts - and the hitman will never kill you as a result. That would be a huge mistake on all the financially motivated holders of yes contracts though - their positions go from worth something (if they sell to the aspiring hitman) to worth nothing if they don't price them low enough. In general you should expect the market to find the price at which a hitman will carry out the contract - so long as there's enough money in the market in the first place.
* Ignoring transaction fees and the time value of money, it's close enough for this discussion.
