For someone interested in learning both Japanese and Mandarin, how much overlap is there? How many more characters would you have to learn to know the 2000 most common hanzi?
Using 2,136 as a reference number (total number of Jōyō kanji)
There are 3,079 unique* characters which form the 2,136 most frequent Mainland Chinese + Taiwan Chinese characters.
- 1567 Jōyō Kanji are part of these 3,079 characters, while 569 are not*:
- 1113 characters of the 2,136 most frequent Mainland Chinese characters are also part of the Jōyō kanji
- 1395 characters of the 2,136 most frequent Taiwan Chinese characters are also part of the Jōyō kanji
Data mined from:
*Not part of or unique here means that they are mapped to different Unicode codepoints. This means that
- Some minor variations, such as Simplified Chinese radical differences (証 vs. 证) are counted as different characters;
- Some minor variations, such as the Shinjitai-unique characters that are mapped identically onto Traditional Chinese, are counted as the same character.
The total number of Jōyō kanji in Japan is 2136.
6335 kanji was used in publications in China, according to research in 1975 and 1976, and 2400 kanji accounted for 99％ of them. So we should think the total number of Jōyō kanji in China is 2400.
Jōyō kanji in Japan same as in China is 1600, so the answer for your question is 1600. However some of them have different meaning and readings, so you may be confused with them.
There is a partial issue with the implications from your question, which namely come from recognition of characters versus pronunciation, and from meaning. There are plenty of kanji that have flat out different readings and meanings from the equivalently written hanzi, meaning you would simply be able to recognize those characters as symbols present also in Japanese. Sorry to be a spoil sport.