Understanding of chipset PCI-Express lanes

The B150 chipset has 8 PCI-e lanes available from the platform controller hub, but there are 16 more lanes available from the CPU.

_{From http://www.intel.co.uk/content/www/uk/en/chipsets/business-chipsets/b150-chipset.html}

Your graphics card would normally be plugged into the first x16 port that comes off the CPU, further devices would come off the ports that remain on the PCH (the B150 controller).

Ethernet, audio, M.2 SSDs and other devices built into the motherboard might take up those remaining 8 ports, but as your graphics card is on the x16 slot from the CPU you won't really care too much and should be fine to use a 970 with this motherboard.

As an example the ASUS B150 Pro motherboard manual shows

Expansion slots
- 1 x PCI Express 3.0/2.0 x16 slot (@ x16 mode)
- 1 x PCI Express 3.0/2.0 x16 slot (max @ x4 mode)
- 2 x PCI Express 3.0/2.0 x1 slots

If you only had 8 lanes available then that first slot wouldn't be listed as being "@ x16 mode".