ELI5: What happens when a lot of people decide to take a short position?

So using snapchat as an example , what happens if literally 90% of the positions are being shorted? Can the transaction go through if I execute it on questrade now? My understanding is that when you short, you're borrowing shares to sell. So .. is there a limited amount of shares that can be shorted technically? Does the share need to be covered buy a certain time frame or can I take a short position indefinitely until the company gets delisted , lol

What does lock up mean? I've seen that phrase float around.

Lastly if everyone is shorting, I should have no trouble buying to cover at whatever the market value is at the time correct?