Did the number of slaves in the US tend to grow over time, or level off? Did these factors change in the lead up to and during the Civil War?
It very much depended on the state and region you're asking about. Here are a couple of older threads on this that may be of interest to you:
https://www.reddit.com/r/AskHistorians/comments/2200dc/if_i_was_an_average_american_citizen_either/
and
Looking at that second thread may be illustrative of the regional differences: only three percent of households in Delaware owned an enslaved person, while 49 percent of households in Mississippi did. Across the enslaving states, about 26 percent of households kept enslaved people.