getNumberImages() isn't documented here but it is available. You can look at ImageMagick documentation for use.
(PECL gmagick >= Unknown)
$GmagickDraw
,$x
,$y
,$angle
,$text
$color
,$width
,$height
,$inner_bevel
,$outer_bevel
$blackPoint
,$gamma
,$whitePoint
,$channel
= Gmagick::CHANNEL_DEFAULT$numColors
,$colorspace
,$treeDepth
,$dither
,$measureError
getNumberImages() isn't documented here but it is available. You can look at ImageMagick documentation for use.