Is it simple to have a blue card in France since i'm holding German one?
It's not exactly simple. You still need to apply with all the usual documentation (diploma, etc.) and a lot of info from your employer (requirements here under “Emploi hautement qualifié"). It's not like a driving license where you hand in your German document and get a French one in exchange.
On the other hand, all this has already been vetted by the German authorities (and their process to check credentials is particularly thorough) so if your job qualifies and your employer provides you with all the required documentation, it should be fine.
What the German Blue card buys you is mostly the possibility to skip the visa application and the need to wait before you are allowed to start working. If any of this should go wrong, you also have the right to return to Germany.
Does the 20 month working and paying tax in Germany counts in France?
If you mean “do they count towards acquiring a national permanent residence status?”, the answer is no. The rules around that are quite different in France and Germany. For example, the requirement to have paid into the social security/retirement system for 60 months doesn't exist in France, what counts is how long you have resided in the country. The time you spent in Germany does count towards gaining the “EU long-term resident” status but that is not exactly the same thing.
Generally speaking, the (permanent) resident status in France (carte de résident and carte de résident permanent) is not very attractive, especially for people who work. It's relatively easy and more common to directly apply for citizenship instead. The main issue with that is of course the risk of losing your original citizenship if you come from a country that has rules to limit the number of dual citizens. Speaking some French is also required.