You can control the camera layout to some extent in
Blue Iris. It will always lay them out in rows in the same order they are organized in the local Blue Iris interface, but you can change that order and change the size of the grid view that is sent to the web interface to make your cameras fit better with less wasted space. If you want fewer cameras per row (and more rows of cameras), reduce the width or increase the height.
Here is where you can change the resolution of those group views.
You will likely need to try many different sizes to find one that fits best and provides good quality for you. For each dimension you should use a number that is a multiple of 16. I used this tool to generate a list of multiples of 16 for your convenience:
Multiples Calculator
736, 752, 768, 784, 800, 816, 832, 848, 864, 880, 896, 912, 928, 944, 960, 976, 992, 1008, 1024, 1040, 1056, 1072, 1088, 1104, 1120, 1136, 1152, 1168, 1184, 1200, 1216, 1232, 1248, 1264, 1280, 1296, 1312, 1328, 1344, 1360, 1376, 1392, 1408, 1424, 1440, 1456, 1472, 1488, 1504, 1520, 1536, 1552, 1568, 1584, 1600, 1616, 1632, 1648, 1664, 1680, 1696, 1712, 1728, 1744, 1760, 1776, 1792, 1808, 1824, 1840, 1856, 1872, 1888, 1904, 1920, 1936, 1952, 1968, 1984, 2000, 2016, 2032, 2048, 2064, 2080, 2096, 2112, 2128, 2144, 2160, 2176, 2192, 2208, 2224, 2240, 2256, 2272, 2288, 2304, 2320
It is easier the more cameras you have, but with enough trial and error you can make a very efficient fill: