A great deal can be said on this issue, but I will give you a quick overview. Please note I am not a sociologist or historian of contemporary Catholicism, just an interested layperson (in every sense of the term).

Yes, the number of women religious or nuns and sisters (see terminological note) in the United States has declined dramatically. It peaked peaked in 1966 at 181,421, but fell precipitously thereafter: 160,931 in 1970, 126,517 in 1980, 102,504 in 1990, 79,814 in 2000, 57,544 in 2010, 48,546 in 2015, and 39,452 in 2021. This is in the context of a self-identified United States Catholic population that increased from about 54 million in 1970 to 73 million in 2021. So there are more Catholics but many fewer women religious. During this period the number of priests in the United States fell as did the number of men in religious life (i.e., monks, friars, Jesuits, etc., most but not all of whom are priests.) (Most statistics from the Center for Applied Research in the Apostolate's FAQ page. But also see CARA's 2014 report "Population Trends Among Religious Institutes of Women.")

We can identify two major causal factors:

• A significant number of departures in the period of about 1966-80, estimated at around 32,000-39,000 in Marie Augusta Neal, Catholic Sisters in Transition (1983). So the main cause of decline during this period was that nuns and sisters were leaving.
• Vastly depressed recruitment of new members since the 1960s. The numbers lost by the departures were never made up and the remaining population of women religious has aged and suffered attrition through death. Just to give an example: the number of women entering religious life in 1958-62 was 32,433. In a similar period of time about two decades later (1976-80), the number was just 2,767.

Taken together, this means that the remaining population of women religious in the United States is aged. According to the 2014 CARA report, in 2009 less than 1 percent of women religious were under 40 while 26 percent were 80 or older, with 80 percent being 60 or older.

Most people who wonder about this don't stop there, of course, and also wonder why so many women religious left in the 1960s-70s and why recruitment fell off. If you're interested in either of those topics, I'm happy to discuss them further and give you some references to and summaries of the literature, popular and scholarly. But I don't want to overanswer your question.

Terminological note: technically, most of the people you may think of and which the general-interest media tends to call "nuns" should be called "(religious) sisters," with the generic term for both being "women religious" and for their organizations "religious institutes." Both nuns and sisters take vows of poverty, celibacy, and obedience, but they tend to live different lives and have different institutional structures. Properly speaking, nuns are women religious who devote themselves to prayer, contemplation, and labor in enclosed convents. Sisters are women religious who devote themselves to active lives of service and thus may have other living arrangements and skllls. For example, Mother Teresa was a sister.