Haka Taka has a good, old guide on managing space on islands, but I thought I'd give an updated one to address the needs of the current meta: legendary and mythic habitats. Other habitats are smaller, so this is less of an issue if you aren't trying to fill everything with legendary/mythic habitats.
The normal habitat cap is 90. Buying the extra island for $15 increases that number to 100 (but only has room for 6 habitats) so the information below works whether you've bought it or not.
Obviously, that totals 102 (100 being the absolute limit), so you've got some wiggle room. But you'll have to keep in mind that your Ultra Breeding Tree and Hatchery are also 6x6 buildings. Your 5x5 buildings are your Breeding Mountain and Vault.
So this main island has a maximum of 19 habitats. As you can see from my arrangement, you've also got room for maximum boosts, and some extra space for additional buildings you might have.
This is to the right and up of your main island. Your maximum habitats here is 6.
This island is directly beneath and to the right of your main island. Maximum here is also 6.
This island is directly beneath and to the left of the main island; it's in 2 sections.
You can get 4 habitats on each section, making 8 total.
This one is directly above and the left of the main island. It only has a max of 5 habitats. Note my Vault is there, but it's a 5x5 building.
This is the center-most top island. It's the top-most island not including the one that costs money. 7 habitats on this one.
Directly beneath the split island. Only 5 here.
Below and to the left of the Island 3 (farthest you can go down and slightly to the right of the main island). 7 habitats here.
This island is in the bottom left corner. 10 habitats here.
Directly above Island 9 (far left and center with the main island). You can get 10 habitats on here. You'll see I've got my Hatchery and Ultra Breeding Tree here. These are 6x6 buildings (same as legendary/mythic habitats).
Bottom-right corner of the screen. Maximum habitats on here is 13.
This is the one in the top left corner that costs actual money to get. You can only fit 6 habitats on there. It's essentially an 18x17 space. I wish it was 1 unit more to be able to fit 3 more habitats, but alas, they didn't do it that way. The good news is, all that extra space can be used for the 5x5 Breeding Mountain or other buildings.