every tile is 32px X 32px
it's 16x16 tiles.
so that's 256 tiles you can play with.
the size of every tileset is 512px X 512px
VX reads a tileset by splitting the tileset in half vertically and setting one half on top of the other. So the first 8 columns wide and 16 rows high is the top half and the remaining 8 columns are the bottom half, when in the editor.
So think about that when you're placing things. You don't want to place like all of your flora tiles all 16 tiles across then import it or when you go to use it, you'll have 8 of those tiles at the top of the editor and 8 more of them somewhere down the middle and you'll be doing a lot of scrolling up and down.
Also, take into consideration what tiles you KNOW you'll be using or ones you KNOW you will NOT be using on other tilesets. Tiles B, C, D and E all share the same passability. Tile A will always be on the bottom or below those tiles, so if you know you won't be using something on tileA, you can replace some tiles there with something that you know will usually be below materials on B, C, D and E. You can do the same with other tiles if tile E gets full.
You can do the same with autotiles that you don't use. Just because the RTP is there, doesn't mean it has to be in your game. For instance, my game doesn't have lava in it anywhere so I removed that and replaced it with a different style of grass that I commonly use in a larger grassy area.