Start with one, first at walk, then at trot. As your horse makes progress, you may go up to three, it will be completely enough to begin with. For useful exercises and information on cavaletti work, I'd suggest this book -
Http://www.amazon.co.uk/gp/product/0851317553/ref=s9_wish_gw_d76_g14_ir02?ie=UTF8&coliid=ILFSTV3WMBPX8&colid=3DGE6KHLDF2SD&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_s=center-2&pf_rd_r=1VNEB1Y7YR7YZPTDXFYE&pf_rd_t=101&pf_rd_p=467128533&pf_rd_i=468294But start from ground, not in saddle, it will be much easier for your horse.
I have come a long way, to surrender my shadow to the shadow of my horse.