You can format numbers as text in LibreOffice Calc. Open the context menu of a cell or range of cells and choose Format Cells - Numbers, then select "Text" from the Category list. Any numbers subsequently entered into the formatted range are interpreted as text. The display of these "numbers" is left-justified, just as with other text.
If you have already entered normal numbers in cells and have afterwards changed the format of the cells to "Text", the numbers will remain normal numbers. They will not be converted. Only numbers entered afterwards, or numbers which are then edited, will become text numbers.
If you decide to enter a number directly as a text string, enter an apostrophe (') first. For example, for years in column headings, you can enter '1999, '2000 and '2001. The apostrophe is not visible in the cell, it only indicates that the entry is to be recognised as text and not as a number. This is useful if, for example, you enter a telephone number or bank account number that begins with a zero (0), because a zero (0) at the beginning of a sequence of digits is removed in normal number formats.