I don't know how many schwas there are when those of all
languages are considered.
In my opinion there are five in my own American English. IPA does not
transcribe sounds quite so narrowly, as far as I know, so IPA would have only
two symbols for all five, either the upside-down e or the lax i symbol.
All are always unstressed and are said so quickly in normal conversation that
it is difficult to be completely sure of the exact quality of each sound.
The same syllable of the same word can sometimes contain one quality of schwa
and sometimes another, even in the same speaker.
1. Nearly equivalent to lax i. ticket, waited, average
Common before t and d.
2. Nearly equivalent to lax u. about, profess, theology
Common in an initial unstressed syllable.
3. Part way between lax i and lax u. wagon, bonus
Common before n and s
4. Nearly equivalent to lax oo, as in book (or u as in push) evil,
table, circulate
Almost exclusively before l, but also in the 'prefix'
to-: today, tomorrow
5. A weakened tense e, nearly lax i: preserve, reply, erase
Almost exclusively in the prefixes e-, be-, de-,
pre-, re-
This is from my own American English pronunciation. Transcriptions of the
speech of other Americans will vary, sometimes considerably, from these.
CJ