Can I add 1x 4G or do I need two x 2G
I would like to upgrade my MacBook (mid 2010 white). But not sure if it has two or 1 slot memory....
Do I need 2 x 2G or can I use a 4G?
First you should make sure your firmware is upto date. Follow this Apple TN: EFI and SMC firmware updates for Intel-based Macs
As to upgrading the RAM follow this IFIXIT guide: RAM Replacement.
And lastly, Here's the information you'll need on the memory from Apple: MacBook: How to remove or install memory. Basically: PC3-8500 DDR3 SDRAM 204pin SODIMM Memory
As it to the max amount of RAM this system will support with the updated firmware you can install upto 16GB of RAM using two 8GB modules depending on your OS (see below note from EveryMac).
*Apple officially supports a maximum of 4 GB of RAM, but third-parties have determined that it actually supports 8 GB of RAM running Mac OS X 10.6 "Snow Leopard" and 16 GB of RAM running OS X 10.7.5 "Lion" or higher and the latest EFI update.