While the actual B450 launch is still two week away, manufacturers are already displaying their motherboards. Gigabyte has listings visible on their website for four B450 models.
Two motherboards are released in ah classic ATX format, two more in the smaller Micro ATX form factor. The ATX solutions are fitted with three PCIe x16 slots, all four mainboards offer 16 lanes for the first slot, the second gets four, the third even just one. The second and third slots are connected PCI Express 2.0, not 3.0. (chipset connection).
The B450 Aorus Pro (WIFI) seems nice with two M.2 slots, but here is a direct limitation again. The first M2 storage slot uses four PCIe lanes, the second is only powered by two lanes. Then again, these are budget offerings. It has six SATA ports for the classic configuration. Only the smallest and probably cheapest Gigabyte board B450M DS3H offers one M.2 and four SATA ports. B450 will be a more budget aimed choice, which makes a lot of sense.