Full length mala necklaces are composed of 108 beads and a guru or finishing bead that unites the two sides, ending in knot work, a tassel or smaller bead grouping. In some mala necklaces, beads of different size or texture may be placed at regular intervals (generally in multiples of 9) to aid the wearer in their mantra practice. Rainework mala necklaces often contain these marker beads every 27th counting bead, and as they function as a marker they do not count in the overall total of 108.
Malas may also be created in smaller sizes and can be created with 18, 27, or 54 counting beads.
How you hold your mala during meditation is a matter of what works for you, though there are certain traditions in specific faiths that you may want to read up on if you are new to the practice. I personally hold my mala in my left hand and loop it easily over my right palm, sliding the beads between my thumb and middle finger as I count.