number - How many of the 613 Mitzvos can we do only in Israel?

There are many Mitzvos we can not do in today's generation. For example there are no Korbonos, no slaves, etc.

Per the Chofetz Chaim, there are 77 positive Mitzvos and 194 negative Mitzvos which can be observed outside of Eretz Yisroel today. How many more can you keep if you live in Eretz Yisroel?

Answer

The Chinuch, in his introductory letter, breaks it down like this (apparently differently than the Chofetz Chaim):

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  There are 613 Mizvot, 248 Positive and 365 Negative. (The Chinuch bases his sefer on the Rambam's enumeration of the Mitzvot.)

  
  


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  The Total amount of Mitzvot that a person can do these days is 369, however 99 of them (78 Positive and 21 Negative), while still applicable, will not necessarily ever be done, since one may go through their whole life without encountering a situation where that Mitzvah applies. (e.g. paying a day laborer on that day, if someone never hires a worker, he will never have an opportunity to fulfill this Mitzvah)

  
  


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  This leaves 270 Mitzvot (48 Positive and 222 Negative) The one will always do, without having to look for situations in which a Mitzvah would apply. The mnemonic for this is "‫אני‬ ‫ישנה ולבי ע״ר‬" [I'm asleep but my heart is awake - in other places it explains that this refers to exile, that even while in a state of sleep (i.e. exile) we still do (at least) 270 Mitzvot]

  
  


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  However, these Mitzvot are not applicable all the time (e.g. Matzot are only a Mitvah to eat on Pesach). There are 6 Mitzvot that are applicable all the time.

  
  

Although the Chinuch does not enumerate the Mitzvot in his introduction, I think it's safe to say that the Mitzvot that can only be fulfilled in the Land of Israel are part of the 99 Mitzvot mentioned in the second bullet point. I don't know how many of those 99 are dependent on the land, though.

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To answer your question according to the Chofetz Chaim, see page 5 of the pdf linked to in the question. In that edition of the sefer (published in 1968), they added a list of the Mitzvot dependent on the land by Yehuda Eizenberg (I don't know who that is). His list adds an additional 37 Mitzvot that are dependent on the land (In addition to the 271 Mitzvot that are applicable outside the land).