Adding line numbers to Docusaurus 2 CodeBlocks using only CSS

custom.css

/* Edit the `src/css/custom.css` file and add the following styles */

/*
 * Reset the line-number counter for each .prism-code scope
 */
.prism-code {
  counter-reset: line-number;
}

/*
 * Notice the chained .language-ts class name to .prism-code
 * You can chain more languages in order to add line numbers
 */
.prism-code.language-ts .token-line::before {
  counter-increment: line-number;
  content: counter(line-number);
  margin-right: calc(var(--ifm-pre-padding) * 1.5);
  text-align: right;
  min-width: 2.5rem;
  display: inline-block;
  opacity: .3;
  position: sticky;
  left: var(--ifm-pre-padding);
}

A variant with display: list-item:

@counter-style codeblock-line {
  system: extends decimal;
  suffix: ""
}

.prism-code {
  list-style: codeblock-line;
}

.prism-code .token-line::marker {
  color: var(--ifm-color-gray-700);
}

.prism-code .token-line {
  display: list-item;
  padding-left: var(--ifm-pre-padding);
  margin-left: var(--ifm-global-spacing);
}

Another even shorter variant:

.prism-code {
  counter-reset: line-number;
}

.prism-code .token-line::marker {
  color: var(--ifm-color-gray-700);
  content: counter(line-number);
}

.prism-code .token-line {
  counter-increment: line-number;
  display: list-item;
  padding-left: var(--ifm-pre-padding);
  margin-left: var(--ifm-global-spacing);
}

https://docusaurus.io/docs/markdown-features/code-blocks#line-numbering
I think you can add this property to the code-block to be able to see line numbers.