By default, Numbers formats table cells automatically. If you enter only a numeric character, it assumes it is a number and displays it accordingly. Entries that look like durations are automatically displayed under the most appropriate duration format.
If you want automatic formatting not to apply to certain cells, you should set the formats for those cells using the Data Format option under the Format -> Cell sidebar menu, preferably before starting to enter the data.
If you are entering movie titles, for example, format the cells to hold the titles as Text beforehand. If you do that, neither "48 hours" will be recognized and displayed as a duration nor "21" will be displayed as a right-aligned number.
Another way to make sure what you are entering in a cell is recognized as text even without formatting is to type a single quote ' first.
See the “Format dates, currency, and more in Numbers on Mac” Apple support webpage for more.