Is 500k enough for tourney? If not, what is?

Too many maybe.

Spies efficacy is based on SPAL, or "spies per acre of land." 500k on the kind of land you'll probably get in tourney is a lot for the environment.

Good to know, thanks Vic.

It's a matter partly of taste and partly of playing style.

So let us say you like to play all-X casher. You will hang back and won't want to be fat if you can help it. Now you would quite like to have high SPAL so as to make it hard for aggressors to spy you. But you also want to have low expenses. Which is likely to be the more significant aspect. So you will get by with minimal spies just as you hope to get by with minimal units.

But then let us say you are a big, aggressive techer. You hate to fail spy ops and you make enemies. You really need high SPAL both to reduce teching turns wasted to failed ops and to defend yourself against adverse spy ops on the part of one or more of those enemies.

You pays your money and you takes your choice. There is simply no one size fits all.