I have 5. One for each of the main 5 Sailor Moon characters as my island is Sailor Moon themed. Now, unlike most people who have more than one character that they don't share with a family member, friend etc. I actually play as all 5 as I don't have a main character and I try to play as them all as equally as I can.
Before the game came out I totally had a plan and ideas for 8 characters but that's super hard to maintain especially when I play as all my characters instead of just using them for storage and an extra house to decorate. Also when it comes to Sailor Moon there's clearly a main 5, and then an additional 5 more supporting characters (6 if you include Tuxedo Mask) so if I did do more than the main 5 I'd only be able to make 3 of the 5/6 additional supporting characters which would make for a really tough choice. I also like having more space on my island to work with with only 5 houses.