IT WILL NOT BE EASY to find another Nancy Hanks to run the National Endowment for the Arts and that's probably as it shoule be, for the challenge now confronting the endowments is quite different from the one that Miss Hanks has handled with such admirable skill and energy. During her eight years on the job, Miss Hanks brought the NEA from relative obscurity to a position of great influence over a wide range of cultural activity in this country. Music, dance, drama, handcrafts, graphic art, the cinema, museums - all these have been nourished by a rising flow of federal grants awarded and administered by NEA under Miss Hank's guiding hand. The endowments budget has soared from little more than $12 million in 1969 to almost $114 million in 1977. The increase in the number of grants has been equally dramatic from 711 in 1969 to more than 5,000 this year. An impressive network of state arts agencies has been developed, providing funds to thousands of local performing-arts and fine-arts groups and individuals.

As the quantity of NEA beneficiaries has increased, so has the quality - and the variety, NEA has reached beyound the better established, familiar and conventional cultural institutions to bring federal assistance to new, experimental activities and organizations, and to community-oriented projects. All this has largely been Nancy Hank's doing, and she made it look easy, as she threaded her way through the cultural minefields in Congress and the bureaucracies with precision and sophistication. By her skill and hard work she has made the NEA a big and flourishing fixture on the American cultural landscape, and all thoese who are involved in or merely care for the development of the arts in this country are heavily in her debt.

But just because NEA growth has been so considerable, and its accomplishments under Miss Hanks so impressive, her departure probably will serve, more or less, to mark the end of an era of expansion - an era that saw an explosion of federally assisted artistic activity across the country. What now lies ahead, we would suspect, is likely to be something of a breathing spell, devoted largely to the consolidation of the gains that have been made and the refinement of the endowment's programs and purposes. The obvious reason is that the amount of federal money available for subsidizing the arts is not going to expand at anything like the dizzying rate of the past eight years. And it shouldn't be expected to. Without being too arbitrary about it, it seems to us that there ought to be some cut-off point beyoud which it is imprudent for the federal government to move in this field - some point at which the incentive to private patronage and support is diminished by too-easy access to the Treasury. There may well be a point, too, when excessive NEA expansion could stir damaging congressional resentment.

The demand for NEA funds, however, is not likely to slow down, with the result that Miss Hank's successor may find the choices even harder to make and the priorities more difficult to fix than has been the case during the big expansion years. This probably puts a premium on finding new ways to encourage private support of state and local arts programs. The new chairman may also want to explore the possibility of making more readily available to local sponsors and managers of activities some of the technical and managerial expertise acquired by the endowment over the years.

The names of several candidates for the NEA chair have been floated about, and one senses that the campaigning, while restrained on the surface, is intense. Politics is no stranger to the talented, temperamental, strong-minded and passionate people that make up what is innocently referred to variously as the art world or the cultural community. So, for that reason alone, we would suspect that the administration's choice will not be an easy one. We trust that it will be taken with some considerable care - for the good of the new chairman, as well as the cause of federal support for the arts. The performance of Nancy Hanks atNEA is not going to be an easy act for anyone to follow.