How many inmates are serving life sentences in the us?

This depends on the jurisdiction and the case in question. No general answer is possible. There are 50 states, various territories and the federal government each with their own laws.

For more than a decade, following the lead of states like California, most states now have statutes that mandate "life means life", and you must serve a minimum of 85% of your sentence on lesser crimes. It is only convicts sentenced on older guidelines that have the possibility(near zero) of a parole hearing yielding freedom after 25 years. This applies to the feds as well - the statutes of most(all?) states are generally poor copies of the original Federal ones.

A life sentence in the State of Florida means that the convict will never be free, but in other states it is 25 years.