From www.Salagram.net

Auspicious Numbers of Salagrams:
1 = Pradhan
6 = cluster (kshetram)
12 = Kshetram
108 = Tirtha
1000+ = Dham (Hari-dham)

The General Rule:
1 is fine.
2 & 3 are inauspicious
4 and any even number after that is acceptable

Odd numbers (except 1) are considered unacceptable

If you have 2 silas (such as I do), and you want to worship both, worship them separately, or as an alternative, include a murti of Lord Krsna to bring the number to the “general rule” as stated above.
Read the other notes included on the page linked below.

(http://www.salagram.net/sstp-howmany2hav.html)