A common answer to any accessibility question is it depends.

This is because there's rarely a one-size fits all answer and often the context around the question will change what the answer might be.

However, I recently shared the WebAIM WCAG checklist with my team and reading through the list I came across quite a few hard numbers which are useful to be aware of:

3:1

The contrast ratio required between a link and the surrounding text if color alone is used to distinguish the link.
1.4.1 Use of colour (Level A)

The minimum contrast ratio for large text (24px regular or 18.66px bold)
1.4.3 Contrast (Level AA)

The minimum contrast ratio for graphic objects such as icons or components in charts or graphs and interface components such as buttons and form controls.
1.4.11 Non-text contrast (Level AA)

The minimum contrast ratio for the various states of interactive components (e.g. hover, focus, active).
1.4.11 Non-text contrast (Level AA)

3 seconds

Audio that plays for more than 3 seconds required a mechanism to stop, pause, mute or adjust its volume.
1.4.2 Audio control (Level A)

4.5:1

The minimum contrast ratio for text and images of text
1.4.3 Contrast Minimum (Level AA)

The minimum contrast ratio for large text to meet AAA standard (24px regular or 18.66px bold)
1.4.3 Contrast Enhanced (Level AAA)

7:1

The minimum contrast ratio for text and images of text to meet AAA standard
1.4.3 Contrast Enhanced (Level AAA)

200%

A web page should still be functional when zoomed in up to 200%
1.4.4 Resize text (Level AA)

80 characters

Blocks of text are no more than 80 characters wide
1.4.8 Visual presentation (Level AAA)

1/2

The minimum line spacing required for text (e.g. size 16px text needs 8px of line-height)
1.4.8 Visual presentation (Level AAA)

1.5

The minimum paragraph spacing should be at least 1.5 times line spacing (e.g. text with 8px of line-height needs 12px of spacing)
1.4.8 Visual presentation (Level AAA)

No loss of content should occur if the user changes line-height to 1.5 times the font size
1.4.12 Text spacing (Level AA)

0.12

No loss of content should occur if the user changes letter spacing to 0.12 times the font size
1.4.12 Text spacing (Level AA)

0.16

No loss of content should occur if the user changes word spacing to 0.16 times the font size
1.4.12 Text spacing (Level AA)

2

No loss of content should occur if the user changes paragraph spacing to 2 times the font size
1.4.12 Text spacing (Level AA)

320px

No loss of content of functionality should occur and horizontal scrolling should be avoided when the content width is 320px
1.4.10 Reflow (Level AA)

This requires responsive design for most web sites. This is best tested by setting the
browser window to 1280 pixels wide and then zooming the page content to 400%

20 hours

An option is given to turn off, adjust or extent a time limit for a page or application unless the time limit is absolutely required or if the time limit is greater than 20 hours
2.2.1 Timing adjustable (Level A)

Users must be warned of any timeout which could result in data loss unless the data is held for longer than 20 hours of user inactivity.
2.2.6 Timeouts (Level AAA)

5 seconds

Automatically moving, blinking, or scrolling content (such as carousels, marquees, or
animations) that lasts longer than 5 seconds can be paused, stopped, or hidden by the user
2.2.2 Pause, stop, hide (Level A)

3

No page content flashes more than 3 times per second
2.3.2 Three flashes (Level A)

44 x 44 pixels

The minimum size of a clickable target unless an alternative is presented, or the target is inline, or the target is not author modified (e.g. a default checkbox)
2.5.5 Target size (Level AAA)

9 years

Content should be understood by someone with 9 years of education, or a more understandable alternative should be provided
3.1.5 Reading level (Level AAA)