I think I actually did some math on this once and determined about 35-50 records per 4x4 depending on whether they are double or single albums/gatefold.
I think conventional wisdom puts it around 45-60 albums per square. Personal experience puts me right in the middle of that range on average of 55 albums, but that includes a mix of slips, gatefolds, 2xLPs, etc. So individual records might be closer to that 60 end.
@gouis You collecting box sets? That would be 21-22 records per square in a 4x4 kallax... and if you intended 4 squares, each square would be packed as shit with 87-88 records each.
I would plan on 40-45 per square, your mileage will vary based off the amount of boxes and 2LPs/gatefolds. I store my box sets separately on a bookshelf, so I can get it closer to 50-55 per square.