A lot of information can be found on the site of the Italian Formula 850 register, especially which builders were working on the Formula 850. From there, further information can usually be found; Google is your friend. In the book "Fiat 850 Fuoriserie" Formula 850 cars are briefly described. I imagine there is a lot more to be found.

A good starting point is the list of manufacturers and their cars.

Below is an alphabetical list of the Formula 850 manufacturers who applied for the certificate of origin from the CSAI at the time and the number of copies certified by each.

It must be said, however, that many cars have been certified several times; this happened in the vast majority of cases because the owners modernized the cars with new bodies, new and higher roll-over bars, modified the suspensions and chassis, even shortened the wheelbase, all this always required new certification.
So how many have actually been built cannot be determined.

Manufacturer/Team Cars certified/built In the book  "Fuoriserie"
A.M.S. 4 Yes
A.T.E.CAR 2 No
A61 1 No
AGOSTI 1 Yes
ALESSANDRINI 1 No
ANDREOLI 1 No
AUTOSPORT 1 No
AUTOTECNICA 1 Yes
AZ 2 Yes
B.W.A. 7 No
BARELLI 2 No
BELLASI 2 Yes
BIGNAMI 2 No
BIRAGHI 9 Yes
BIREL 1 Yes
BONETTI 3 Yes
BRANCA 1 No
BROGAM 1 No
BUGLIONI 1 No
C.R.M. 1 No
C.V.R. 2 No
CARENA 1 No
CICERI GMS 2 No
D.R.M. 1 No
DAGRADA 5 Yes
DALGO 1 No
DE SANCTIS 29 Yes
DEALGO 1 No
DEL GIOVANE 2 Yes
DEMA 1 Yes
DIANA 1 No
E.L.E. 1 No
FAGIOLI 7 Yes
FALCONE 1 No
FELISATI 1 No
FERRAZZI 1 No
FIDE 2 No
FIORETTI 1 Yes
FOGLIETTI 1 Yes
FRABER 1 No
GDS 1 No
GIANNINI 1 No
GIGLIO 1 No
GILI 1 No
GIULIANI 2 Yes
GPD 1 No
GRASSELLI 2 No
GSC 1 No
GSM 1 No
GUARNIERI 1 Yes
IPPOCAMPO 2 No
JACO 1 No
L.G.A. 1 No
L.P.T. 1 No
LUCKY 11 Yes
M.Z. 8 No
MACO 2 No
MANTISSA 1 Yes
MARRAS 1 Yes
MONTAGNA 1 No
MONTEVERDE 1 No
NEISSE 1 No
NIPAMA 1 Yes
OL-GA 1 Yes
PAPARUSSO 1 Yes
PATRIARCA 1 Yes
PRAT 1 Yes
PRATALI 1 No
PULVIRENTI-BRACCINI-PT 1 No
RENZINI 2 Yes
REPETTO 3 Yes
RESIDORI 1 No
RRE 1 No
SACE 1 No
SANETTI 2 No
SCA-BRU’ 1 No
SELENE 3 No
SILVA 1 Yes
SINIBALDI 1 No
SPICAR 1 No
STELLINA 1 No
STYLAUTO 1 Yes
TABI V.D.R. 1 No
TAGLIANI 2 Yes
TECNO 13 Yes
TONY KART 4 Yes
VACCARI 2 Yes
VIALI 1 No
VICO 1 Yes
VIGEZZI 1 No
WAINER 2 Yes
Z.A.M.B.A. 1 No
ZANI 2 No
 Total certified 205