Most manual wind watches will have a standard complement of 17 jewels.
Automatic winding movements will add about 4-8 jewels to help most efficiently transfer the relatively small rotor forces into winding the mainspring. An automatic Winding watch will normally have 25 jewels.
Special Movements with additional functions such as chronographs and calendars, can up the total number of jewels tremendously. For example, the IWC Il Destriero Scafusia (claimed to be among the most complicated wristwatches) has a total of 76 jewels to accommodate the time, perpetual calendar, rattrapante chronograph, repeater, and tourbillon functions - and this is a manual wind watch.
Why do they use synthetic ruby?
Ruby is significantly better than steel in handling the forces involved, never wear out easily and providing a super low friction surface suitable for both high-load as well as high-speed motion. With modern production methods, they are cheap USD$0.1
Ruby is technically known corundum, and is a crystallized form of aluminum oxide (Al2O3). In pure form, corundum is white in colour; trace impurities are added to change the colour - to red in the case of rubies. It should also be noted that any other colour of corundum (including clear) is known as sapphire. Ruby is used because it is an extremely hard and provides a smooth surface for the wheel pivots (and other steel components) to operate on. In a mechanical watch, there is a constant force applied to the pivot of every wheel in the wheel train, which is applied by the wound-up mainspring… Without any jewels, the steel wheel pivots would very quickly wear away the bridge and plate material until the wheels came out of alignment, and the movement would crash to a halt. In the inexpensive watch of yesteryear, the pivot holes may have been provided with hardened metal bushings.
Are more jewels better?
Not necessarily. As noted above, a typical hand-wind movement today will have only 17 jewels as a full complement. Some really high-grade or ultra-thin movements will add a few extra jewels to further protect against any wear, but even these top out at 21-23 jewels.
Only those pieces of the movement which are between the mainspring and the escape wheel are candidates for jewelling, as these are the movement parts that experience the higher forces or relatively higher speeds of the mainspring or escapement. Other components, such as the motion works (i.e. hour and minute wheels), calendar mechanisms, and winding train are not under this constant stress, and thus arguably do not need jewels.
One will occasionally encounter a quartz movement with 4-6 jewels in it.
As a historical note, there was a "jewel craze" about 50 years ago, where manufacturers, under the belief that the public thought more was always better, came up with 75 or even 100 jewel movements. They are decorative Jewels.