If you buy it at 20000$ for sure its a big upset on you side because you loss more that 50% of your capital.
Can you Make an Advice When I should Buy Bitcoin?
And how much money should I invest on it?
Of course, everybody will be disappointed when looking at losses that exceed 50% of funds used, the last time when bitcoin prices have reached $10,000 and it didn't last long even within a week.
The last time I read an article from Pantera Capital, dropped bitcoin price since the beginning of 2018 was caused by taxation that forced sell a lot of bitcoin to meet the tax requirements. And They also explain that in 12 months bitcoin prices will be back at $20,000.
My suggestion is better to invest your money 50% of current price and wait within 12 months hopefully the article gives accurate news.
The price of Bitcoin is not going down further but is below 10k dollar which is achievable and one more thing which is important to highlight is the constant increase of Bitcoin value from 7k dollar to 7.7k dollar, if this constant increase continues then it is the sign to enter into a green market. We have seen the low markets before those recovered again and made us able to get profit. All you need in this time is to utilize it and to invest in this low market which is favorable for buyers.