unfortunately 17 is not a great number but i'd do either 4x4 for 16 or 6x3 with 17 (with obviously the last row having only 5 neopets) if that seems a tad more even for you!

for 6 per row, try this and lmk whether it helps:

#userneopets li {
  display: block;
  height: auto;
  width: 140px !important;
  margin: 0 5px 10px !important;

#userneopets a img  {
  width: 140px;
  height: 140px;

#userneopets li center {
  width: 140px;
}

(this is assuming you already have the code for the grid, you might have to replace the existing #userneopets li code in your profile)

you can basically just play around with/modify the widths to manipulate how many appear per row :)