The first technical preview of Windows 10 for Phones was limited for a small number of phones. Now the Microsoft announced that another 36 phones will be getting it on the next release. They may add more devices to the list before the release. They are testing the devices and if they found any bug they will not be releasing Windows 10 for that device.
Here is the List of Windows phones expected to get the next Windows 10 Technical Preview for phones
Lumia 530 Dual Sim
Lumia 630 Dual Sim
Lumia 730 Dual SIM
Microsoft Lumia 430
Microsoft Lumia 435
Microsoft Lumia 435 Dual SIM
Microsoft Lumia 435 Dual SIM DTV
Microsoft Lumia 532
Microsoft Lumia 532 Dual SIM
Microsoft Lumia 640 Dual SIM
Microsoft Lumia 535 Dual SIM
If you have any one of these devices and wanted to test Windows 10 on your device, you will have to register to the Windows Insider program and install the application from the Windows store. You can read more detailed steps from the Microsoft Windows website here
I own a Lumia 1320 and waiting eagerly for the next release.