The number of meetings is not stipulated and Celebrants differ in this regard. However, from a reasonable perspective, you would need to meet at least twice with your Celebrant unless this isn't practical e.g. if you live interstate or overseas from your celebrant. The first meeting may cover the signing of the Notice of Intended Marriage form, the provision of contact details etc. and various other matters such as a preliminary exchange of ideas about the type of ceremony you would like to have and what your Celebrant can provide. Another meeting would be held quite close to your wedding date at a mutually convenient time. At this meeting, you will be required to sign a document referred to as the No Legal Impediment to Marry, literally a statutory declaration that states that there is no legal impediment to your marrying one another.
Apart from this, the second meeting allows you to perhaps refine your ceremony and/or any of its staging aspects as well as practising certain parts of the ceremony e.g. the exchange of vows and the exchange of rings, to pay the balance of the Celebrant's fee (if you have not already done so) and generally attend to any other outstanding matters.