The main causes of de novo mutations are mutagens including UV, Gamma, X-rays, chemicals and viruses. These mutagens can tear DNA, change DNA codes or cause cell to make a mistake in DNA replication

The human genome mutation rate is similarly estimated to be ~1.1×10−8 per site per generation...

It depends probably of the environment.

The answer also depends on the cells used for sequencing. Some of the numbers cited above are valid for germ cells, somatic cells will have a range of mutation rates, some are even known to mutate in a semi-programmed fashion via recombination mechanisms and transposons.

Of course individual cells will accumulate mutations over time (otherwise we would not be troubled by things like cancer).

But detecting them might be tricky: As one usually determines the genome sequence of a (small) cell population (from a biopsy sample, a swab, etc.), individual mutations well most often be "masked" by the population. And as Matej stated, the source of your cells might account for most of the mutations one observes.

I do not know whether it has been studied how many mutations can be observed in a single individual in cells from different tissues...

A burgeoning perspective is on the importance of common structural rearrangements: http://www.sciencemag.org/content/342/6158/564.summary