Too many unprocessed floats

You can use the command \clearpage. This prints all floats that are not yet placed and starts a new page.

Try the morefloats package. From the README:

LaTeX can, by default, only cope with 18 outstanding floats; any more, and you get the error "too many unprocessed floats". This package releases the limit; TeX itself imposes limits (which are interdependent of the help offered by e-TeX). However, if your floats can’t be placed anywhere, extending the number of floats merely delays the arrival of the inevitable error message.

For LaTeX releases from 2015/01/01 onwards you can allocate more (several thousand more) floats than the standard 18, using a command such as

\extrafloats{100}

in the preamble. As this mechanism uses the etex extended box registers it can allocate many more float boxes than the morefloats package mechanism (which uses \newinsert and is thus restricted to the classic TeX registers 0-255).

If for some reason you are using a LaTeX format built on tex rather than etex, \extrafloats is still defined, but falls back to using \newinsert so fewer float boxes are available.

Edit, from the 2015/10/01 LaTeX release the default number of floats is increased from 18 to 52, and these reserved floats may also be used by \newinsert once the classic TeX registers run out.