You can put as many disks into RAID 0 as you have ports to connect the drives with. But the more drives you add, the greater chance you have for a disk failure and if ANY disk dies then you loose ALL your data.
And while you can improve the transfer rate by adding drives, you can't really improve the access times, and access times are what matter most for general-purpose performance when doing things such as booting the system or starting applications.
You won't see any significant performance improvement with SAS, the limiting factor is the mechanics of the disk itself and not the communication protocol. The only way to get a benefit from SAS would be to buy higher-performing drives such as a 15Krpm enterprise-class drive which is available in SAS but not SATA - but those can be quite pricey and if you're really looking at that kind of solution then you should seriously consider an SSD.