SSD device capability may be in 2,5" or m.2 form.
It depends on the particular laptop.
All will have at least one.
Modern laptops will have at least a m.2 slot because those devices are smaller and lighter.
Many will have a second space in either m.2 or 2." format for expansion.
As to how many you need, the answer is likely just one.
You can put some very large capacity devices in just one mounting position.
There is no real performance impact from using just one device.
I might add that when buying a laptop, the upcharge for more ram or higher capacity drives is significant. Usually2x the retail cost or more.
If you are comfortable making updates, buy the minimum ram and drive capacity and plan on upgrading yourself