How many exams can one take in a year. I am asking this because I have to take the theory grade 5 exam before taking the piano grade 8 exam, but I already play some pieces of the LRSM. Would it take me at least 2 years before being able to take the Dip ABRSM?

You can take as many as you like but you must have passed the grade 5 theory before you enter the grade 8 and you must have passed grade 8 before you take the Diploma.

sure, I understand that. But will it have to be this way: in 2011 I take the grade 5 theory. I wait until 2012 to take the grade 8 piano. Next, I wait until 2013 for the DipABRSM, and so on? Or can I take the grade 5 theory in 6 months and in the next month take the grade 8 piano examn, and in six moht take the dipabrsm, etc?

It depends on where you take them. In the UK there are 3 sessions, Spring, Summer and Autumn and you could do one in each session.
Internationally there is usually only one exam session for practical exams per country per year and maybe more for theory.
I am in Austria and they have 3 dates for theory exams in the year (mine is next week!) and one session for practical May/June. Germany has two sessions for practicals - March/April and another later in the year.
So it depends on where you are going to be taking your exams.
You could not take your grade 8 and Dip in the same session because you have to have passed grade 8 first before you are allowed to enter the dip.

If there is only one session per year in the country you would be in, then it might be worth speaking to the board about doing G5 theory and G8 practical at the same session.
This has been allowed before, basically your G8 practical result will be contingent on the G5 theory pass. If you failed the G5 theory, then the G8 practical wouldn't be awarded no matter how good a result you got.

If you have more than one session in a year, then you can take them in consecutive sessions within the year, you don't have to wait until the next year.