Random luck or by units?
It seems like I need to use 6 turns in order to have one successful op.
I had 180k spies and target has merely 40k with no tech.
We both running the same country

Does bonus adds up on spy defense?

It's a ratio. It was based on spies/land in the past.

I believe regular spy ops and harmful ops are handled differently.
There is such a thing as spy dr, which would make it much more difficult to succeed on a country that had been "spied out".

Only "bonus" i'm aware that would effect this would be the Dictator spy bonus, and technology.

Here's my experience so far from experimenting: (And my experience is limited to the EXPRESS server only)

Information spying is significantly easier to achieve. (ie, market spy, military spy, spy)

It takes approximately 30 non-information SPY OPS to spy a country out. (Meaning after 30 spy attacks you'll notice a significant number of failed SPY OPS to the extend that all your non-information spy attacks fail).

Information spying is unaffected by a spy OUT country.

As Kingme has noted before, SPY 'bonuses' are limited to spy tech and dictator spy bonus (30%). If a country has both, the two #s are multiplied. (ie. 1.30 * 1.40) Then that number is divided by the # of acres. That gives you your SPAL.

Yes. And because effectiveness operates off the spies per acre of land ratio, it means a country with a relatively small number of spies but very little land can easily outspy (or defend against spy attacks from) a country with a larger number of spies but also a lot of land.

Thanks.