2020-10-28 Note: seems like arma3 v2.0 update has fixed this issue as at least we can load much more roads.shp features without a crash (tested with 19,212 so far). Please report back to us if you are still getting this crash described below.
When you get arma3_x64.exe crash with too many virtual blocks requested it means (most likely) that you have too many roads in the shapefile.
Too many roads means too many shapefile features. Smallest road feature is polyline with 2 vertices, however larger roads can be huge in the vertices count.
It apparently is counted from the terrain data as whole, because smooth iraqi terrains work fine with 9000 road features but rough afghanistan terrain can crash even below 5000.
Good rule of thumb is to keep the road shapefile feature count as low as possible, with smaller terrains this is not an issue but when you do large terrains with huge road networks, you might have to do some hard cutbacks on the road count.
Too many virtual blocks requested crash can happen also when you have too many addons loaded. Often times we see people just load every addon to arma3 without having no idea what it might cause.
Just because there are plenty of great addons for arma3 it doesn't mean you can or should load each and every one of them at the same time.
Frost in PMC discord server provided us with list of addons that cause issues on PMC real world data terrains. Below is the chat log:
Frost: ACE, ACEX, ACRE2, CBA, ALiVE, Ambient Battle Sounds, 2035 RAF, CSAT Winter Units, CUP Factions African and Asian, CUP Factions, CUP Weapons, CUP Vehicles, Theseus, RHSUSAF only, Cyprus Push to hear, Enhanced Movement, Shacktac UI, Advanced Urban Rappelling, JSRS Sound Mod, Advanced Train Simulator (not sure why I still have that enabled, actually…), JNS Skycranes, Pook AFV and Camonets, XENOTaruPod Mod, NIArms Core and AK pack, SMA
Frost: There are a few others, but they're just client-side stuff. Hm, oh, and some editory stuff - 3Den Enhanced
Frost: CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad : OK
CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad, ACE (with CUP compats) : OK
CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad, RHS USAF : OK
CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad, ACE, RHS USAF : CRASH
CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad, ALiVE : CRASH
CBA, CUP Weapons, Units, Vehicles, Terrain, PMC Terrain Data, PMC Jalalabad, ACE + Almost anything else : CRASH
Conclusion: only start your terrain with the absolutely minimum required addons, this way you can be sure the crash is not caused by too many (or even faulty) addons.