If your employer sponsors your NIW petition, a new petition may have to be filed. If you petition for yourself, changing employers should not affect the status of the case. The new position must continue to satisfy the requirements of the NIW.
Generally, it is suggested to submit four to six letters when petitioning for NIW, but there is no specific number.
No, this varies because it is all determined by the USCIS on a case-by-case basis.
You may self-petition for NIW or an employer may file on your behalf.