I'm going to run a TSoY one-shot on Wednesday night, and one of the players told he'd like to spend four of his five starting advances on Keys. That means that his character would start play with five (5) Keys.

How is this? Is it a problem? If so, should I just advise him not to buy so many Keys?

Or, is it a great opportunity? If so, how do I best take advantage of it?

This may be one single point where the old "don't advance where you didn't last" rule holds.

Just be brutal. Lack of Pool points, as well as Secrets and abilities will quickly lead to a character that grows through pain.

It's simply against the rules. Let him read the "creating a beginning character" -"character Advancement" chapter, where it's written "You can never increase the same thing twice in a row. That is, you cannot increase the same ability or pool twice in a row. You need to increase something else in between. In addition, you cannot buy two Secrets or two Keys in a row. ".

So, at most, a player can spend 3 of 5 initial advances on keys. Not four.

Four or five keys don't make much of a difference. And that rule has gone from Solar System, so it may not be against the rules, depending on the ruleset incarnation used.

For a one shot, I wouldn't mind at all. That player will probably try to churn through keys to make sure he transcends, which would be fine by me.

Paul,
 The only "danger" from such a player is if they intend to buy off most of them for the XPs.

  I wouldn't confront this player with that accusation (are you just buying this key to buy it off), because that might inspire them to do something they hadn't thought of.

  Instead, I would talk to them about there character and what they are like. and if the Keys he buys matches the description they give, then its just a matter of a highly motivated player.

  Really, I had a player who was sitting on 11 advances in my last campaign and it was fine. The reality is, this game has a lot of things for you to spend XPs on, and getting more XPs than another player doesn't make them a better or worse player or character.

  Honestly, I would would encourage a player to take 5 keys, then monitor their play and let them trade Keys for advances if the Keys weren't firing very often. But let them keep it and enjoy the XPs if the player can get them to fire consistently.

Great feedback, thanks!