Depending on the plan that you select, there may be an allowance per line or a set allowance shared between all of the lines, every month
Its important to understand the details of each carriers plan before you sign up.
Shared allowance - some carriers offer plans that have shared data across all lines, meaning from two to however many lines you have the data and or text/talk will be shared.
For example if there are two lines on a shared plan with 5GB of data and 1000 min of talk. If one line uses 100 min. and 1GB of data, and the other used 500 min. and 3GB of data there would be 400 min. left and 1 GB of data left between both of the lines.
Individual line allowance - some carriers offer a per line allowance of voice minutes, texts and data. In this case once the usage has hit the per line limit then you wouldn't be able to access all services - usually this is most relevant to data - usually the data speed would be reduced though so that you could still access basic services at 3G rather than 4G speeds