How would you create 8 violin voice or 4 cello voice from the waverom with only few samples you have?

You cannot determine the number of violins or cellos in the waveform. Sampling here does NOT work like that. You'd have to record the instruments yourself. And even then, you cannot control the exact number playing, easily. Sample playback basics: if you sample 8 violins, this means every note you press reproduces 8 violins, play a two note chord 16 violins, play a three note chord 24 violins and do on... A five note chord is 40 violins.

Is it the introduced phase shift effect that makes them sound funny if I use 8 violins for example?

This depends on the Waveforms you select. The factory strings don't "sound funny" copy what is done in the factory Voice to avoid phasing issues. If you are stacking the same violin Waveform eight times it will not sound like eight violins, not ever.

The thing that 8 violinists CANNOT do, no matter how accomplished, is play exactly the same, with exactly the same phase relationship, and tuning. It is the small but important imperfections that make it an ensemble. All you can do by stacking eight violin Elements is make a funny sound...

Fortunately, when Yamaha had all the string players together we captured the ensemble sound for you. You cannot reproduce this via your method, the same or similar samples stacked that way is wasting polyphony in an attempt to build a sound that will be better if you select a waveform that already is an ensemble.

Because of the way samples multiply the size of the ensemble according to how many notes you play will negate any advantage gained by stacking 8 violins. If each finger could play a single violin, that would work.

Suggested workflow: record your string parts using a solo violin, but record multiple times, each overdub being a single player, no chords... Ala Walter Carlos doing Switched on Bach...

Thanks Phil. I didn't mean to say the samples themselves sound funny but like you said they sound abnormal when mixed that way. I want to create a string sound which is more voluminous/fat(may be). I was not satisfied with (specifically) the mid and higher notes of the preset VOICEs.

The waverom samples on the other hand, sound really good! BUT there are few issues and I want to resolve about the VOICE I made. I was listening to the +/- samples in the waverom and it seemed that (-) samples matched better or more similar to the song I'm trying to cover.

But still, some kind of unpleasant sharp sound in the high notes do not match to what I want. May be the string samples were recorded too perfectly! In the recorded song I can sometimes hear a slightly dominant instrument depending on which group of players is playing. I simulated this by adding 2 single instruments violin and cello which the sometimes play along the section to simulate that.

Thanks for you advice. I'll play with it and I'm sure I can make it sound better than what I got so far. I also realized to using an effect drastically improves the string section's output. I'll update this reply when I got home and ask a question about the effects later.