Generally, 'to what extent' questions should be answered quantitatively i.e to a limited extent, to a significant extent, to the fullest extent etc. It is probably better if you don't simply answer the question with a thesis like "X was a cause for Y to the most significant extent." and instead make your statement stronger by adding something else, possibly a reason for how you achieved that conclusion such as "X was a cause for Y to the most significant extent due to Z." And then your essay would be explaining why you think X was significant to Y, but constructed under the Z framework.
Also, it kind of depends on the essay question. I doubt you will have enough space to talk about all the causes. I definitely think it is a lot more beneficial to focus on few. You should mention more than one if only for comparative purposes because a "to what extent" question typically requires you to quantitatively judge the significance of your causes, and arrive at a conclusion. And your points should be only the most significant ones - they will make the backbone of your essay. There is no point talking about the less significant points if they don't directly contribute to your thesis, otherwise it will damage the clarity of your essay.
You should mention other causes to support your argument. For example, kind of like counterarguments, you can explain how one cause is inferior to the significant cause, or explain how it can actually be categorised under your significant cause.
Now, if I were to argue that X was only a marginally significant cause compared to say, another cause W. I would focus a bulk of my essay on why I think W was the most significant cause, but I would also dedicate a good chunk focusing on X, because the question needs me to talk about X somehow.
I don't do Russia, but this is how I answered a 'To what extent' Q:
To what extent did the policies of Republican administrations contribute to the financial crisis of 1929 and the Great Depression that followed?
The policies of minimal government intervention and the inability to responsibly regulate the economy which was inherent in the Republican administrations of the 1920s, contributed to the Great Crash and Great Depression to a significant extent. While it may be contended that Great Crash and Great Depression were harnessed by factors other than that of the Republican economic policies such as economic dualism and weak banking system, it was in fact the Republican administration that exacerbated these factors.
I hope I didn't confuse you! Good luck