Do you mean generally or around a specific airport?  The number of autogen trees will change depending on the autogen slider in FS9

Replacement trees might be an alternative (backing up originals first). Although created for FS2002, the one I am using ("MSTrees2.zip") works very well in FS2004, and is the one I settled on after having tried several variations of replacement trees. Unsure from where I got it but it ought to be widely available. The following is copied from the header in the readme:

**********************************************************************
                    Gerrish's Trees Library for FS2002
                    Part 1 - Autogen Trees Replacement
                          Version 3, 2002-04-26
                    ----------------------------------
                                    by
                               Gerrish Gray
                        Copyright (C) 2002 G Gray
**********************************************************************

Here is my third alternative replacement for the AutoGen vegetation
texture sheets that provide the trees in the FS2002 scenery (see
GTL_AGEN.ZIP and MSTREES.ZIP for the others). This one is a second
enhancement of the new set recently released by Microsoft as part of the
AutoGen SDK. The number and variety of the trees is increased and
undergrowth has been added to further increase the density of the tree
cover. A number of other details have been modified to improve the
overall appearance. Forests now have up to 3,500 trees per sq km,
compared with the scanty 600 of the original FS2002 release, and the
tropics look more realistically 'tropical' with lush green bush below
the palm fronds.