The number of expansion packs you can install depends on their content. For example, expansion pack can only contain 5 styles and 1 voice -in this case you can install a lot of them. But if expansion pack(s) contain many styles and voices, then it can happen you will only be able to install one single pack.
You combine expansion packs with YEM (Yamaha Expansion Manager), where you can choose what you wish to use from each pack. And when you do that, you can see on top bar, how much memory will be occupied by your selection (PSR-SX700 has ~400MB available space for expansion packs). As you will notice, voices (in packs) are quite memory hungry, so you should only choose the ones that you really wish/need.
Usually, many styles in original expansion packs use custom voices which are provided in particular expansion pack. The problem (with crappy YEM) is, you can't know which voice(s) are needed for particular style. Means, if you only choose style(s) in pack, they will work on keyboard, but won't sound as intended. Without going into "how styles work", let me just say: In some cases, you will be able to see on keyboard, that voice is missing (after installing pack). However, many times, that can't really be seen. For that reason, some simply install all voices in particular pack (just to be sure), which is not that rational decision. On the other hand, many just ignore voices (to save the space in keyboard memory) and are using styles only -without even knowing, that style doesn't sound as it should.
To solve this problem, you'll need some 3rd party "style revoice" software, which can show you, what voices inside pack is particular style using. You can find some in this forum, but I can't point you to some particular, because I'm using software that I made myself. Important: before you simply "revoice" a style (replace missing voices), you should know how the original sounds.
To summarize: yes, you can make "perfect" single custom expansion pack, which only contains things you need from other packs. And additionally, you can also add your custom made styles,voices, pads, registrations, etc. to that pack -the only limit is keyboard memory.
I hope I was of some help,
Bogdan
