one of the most irritating parts of running your own instance is having to modify source files just to change your character limits. there are plenty of posts out there describing how to change the character limits for posts, but almost none on how to change the limit for alt text (1500):

here's the source file where the alt text char limit is buried:
/instance/app/javascript/mastodon/features/ui/components/focal_point_modal.jsx

and here's the source that needs to be changed:

<FormattedMessage id='upload_modal.detect_text' defaultMessage='Detect text from picture' />
</button>
<CharacterCounter max={1500} text={detecting ? '' : description} />
</div>

<Button
type='submit'
disabled={!dirty || detecting || isUploadingThumbnail || length(description) > 1500 || is_changing_upload}
text={intl.formatMessage(is_changing_upload ? messages.applying : messages.apply)}
/>
</form>

change "1500" to whatever value you want.

#mastoAdmin