Guru

Mrs. Meera Tadipatri's explanation

Niyata-guru is:

"shishyasyasvarUpaM saMyajj~nAntvA tadyogya-vishishhTa-bhagavat-svarUpa-vishayaka-vidyA-upadesha-karta"

One who knows shishya's svabhAva fully well [saMyajjnAtva] enough to impart the jIva-specific bhagavat-svarUpa upAsana [to get yogya aparoxa.]

This niyata-guru is svAbhAvically higher, and the tara-tama bhAva aka higher-lower-sambandha continues in moxa too as it is part of the svarUpa itself.

Thus a manushya jiva can only have Rishhi-jIva as its niyata-guru and not a fellow manushya jIva [or even chakravarti jIva!]-- no matter what varna, ashrama, gender superiority he may have.

Thus without pramana one should never say xyz is the niyataguru of abc -- as it violates tAratamya which is a grave sin!

Aniyata-guru

does *only* bhagavat tattva upadesha.

This is only for shravana.

"aniyataguravastu bhagavat-tatva-mAtra-upadeshhtAraH"

Aniyataguru may vary from janma to janma depending on janma-karma-yogyata. Aniyataguru may or may not be svAbhavicaly higher. The "shishya" may already know more than what is being taught.

III. How many VidyaGurus may exist to one person?

In case of niyata-guru, Normally only one per jIva with re to Devatas.

As for aniyata-guru-s, there is no restriction. However, in one janma, there are procedure one has to follow while changing aniyata-guru-s.