It depends on how much diversification you think you need and what your mutual fund options are. For instance, picking an index fund already provides a fair amount of diversification, especially if you select a Total Market type of index (readily available from Fidelity and Vanguard, and many other fund families). Are you looking to balance domestic vs. international investments? You may want to add an international index fund to the mix. Feel that a particular sector has tremendous potential? Add a sector fund. This investment mix is up to you (or your investment advisor).
However, depending on your Roth IRA mutual fund choices, some of these funds may have minimum investment requirement - $3k to open a fund's account, for instance. In that case, you'd have no choice but to put your entire investment into one fund, and wait for subsequent years where you'd be able to invest in other funds after providing additional contributions and/or reallocation any growth from your initial investment.
One thing to look at is whether you have an option of putting some of your contributions into a money market account within the Roth IRA - you can then reallocate funds from that account into another fund after you can meet the minimum investment requirement.
However, in my opinion, if you start out by investing in a solid, low-cost index fund from a reputable mutual fund company, you've already picked up most of the diversification you need - a single fund is enough.