When creating a map and add a host group as the only map element. Then tell zabbix to show "Host group elements".
If the host group has many elements the auto-aligning on the map fails.
I tried a map width between 900 and 1200 (so you wudln't have to scroll sideways) and height between 10000 and 40000.
Zabbix creates just a few rows with items (in my case 28) where everything overlaps while only increasing the space between the rows instead of creating new ones.
I don't know, where the limit is. All I can say is that ~30 elements worked, ~550 failed, unfortunately I've got nothing in between at hand.