You must purchase only one SSL certificate for site. Authorize.net module connects to site with openssl library and creates one internally for you.
Anyway, one certificate is enough and you must install php with SSL support (--with-openssl)
Actually, you don't have to buy a certificate if you want to test it only.
You can generate self-signed certificate (output: me.cer) and install to web server.
Some useful openssl commands:
CREATE PRIVATE KEY:
genrsa -des3 -out priv.key 1024
REMOVE PASSKEY FROM PRIVATE KEY:
rsa -in priv.key -out privnopass.key
CREATE PUBLIC (CERTIFICATE) KEY - SELFSIGNED
req -config ./openssl.con -new -key ./priv.key -x509 -out me.cer -days 365
req -config ./openssl.con -new -key ./privnopass.key -x509 -out me.cer -days 365
SIGN REQUEST ( FOR CAs )
ca -in r.txt -out newcert.pem -config openssl.con -keyfile CA/server.key -cert CA/server.cer