The Surfer worksheet supports up to 1 billion rows by 1 billion columns. Your computer will run out of memory before you can import that many data points.
If you want to find out how many data points you can grid, you can estimate how much RAM you will need (in bytes) by multiplying the number of data points you have by 400. Surfer requires up to 400 bytes of memory for each XYZ data point when gridding. So if you have a 400,000 point data file, you will need 400,000 x 400 = 160,000,000 bytes of free memory (or 160 MB).
If you are running out of available memory when you are trying to grid a data set, please see the options described at: Why do I get an "insufficient memory to perform operation" error when gridding data in Surfer?
Updated January 19, 2017