The number of VPN servers a provider has can have a huge effect on how the service works overall.
Having a large number of servers means that each one will have fewer users, ensuring better speeds, and the wider the variety of IP addresses, the more likely it is that you'll be able to watch Netflix successfully.
Also, the number of VPN server locations is useful, too. With more locations, no matter where you are in the world, you'll be able to connect to a closer server, ensuring better speeds.
However, you'll want a happy medium – just a few servers spread over tons of locations will result in poor speeds, and thousands of servers in just a few countries will be limiting in terms of content available.