This depends on the license of the version of Office you have and into which
partition/s you may be installing it.
If you just are reinstalling into the same computer's partition because you
are rebuilding it there should be no limit but after several reinstalls over
a period of time you may be required to call to explain the situation to
Microsoft (MS). A reinstall within, IIRC, 90 days will also require a call
to MS.
If you are dual or triple booting the machine and installing the program to
each "bootable" machine you will need to read the EULA. Some allow three
installs (Home & Student) but most others will require a new license for
each bootable machine. (The EULA for most other retail versions say you can
install on one device and one portable device used by the primary user so
this would not be in accordance with the EULA to install on two partitions
on the same machine.)
You can search and read the EULAs for all Office software here:
http://www.microsoft.com/about/legal/useterms/
