You have to write custom visual force page to achieve this kind of functionality. Salesforce does not support to view excrete of data or visible only some selected lines. You can create a custom field and populate the n (number of lines) lines from long text field by creating trigger.
Full Answer
How do I display formatted numbers in Lightning component?
A lightning-formatted-number component displays formatted numbers for decimals, currency, and percentages. Use format-style to specify the number style. This component uses the Intl.NumberFormat JavaScript object to format numerical values.
How do I format numbers in Salesforce?
Use format-style to specify the number style. This component uses the Intl.NumberFormat JavaScript object to format numerical values. The locale set in your Salesforce user settings determines where to display spaces, commas, and periods in numbers, and the currency used by default.
How do I change the default decimal places in Lightning-formatted-number?
By default, lightning-formatted-number displays the value 1000.0 as 1000. To display the decimal and trailing zero, set minimum-significant-digits to 5. Customize the number of decimal places displayed using minimum-fraction-digits and maximum-fraction-digits. Decimal numbers default to 3 decimal places.
How do I display the percentage of a number in Salesforce?
Add the attribute maximum-fraction-digits="3" to display the percentage as 0.503%. Large numbers with more than approximately 15 or 16 total digits can't be formatted correctly. In these cases the unformatted value is shown. The locale set in your Salesforce user preferences determines how numbers are formatted.
What is variant in Lightning?
A variant overrides the display density for that field.
What is display density in Lightning Experience?
In Lightning Experience, the display density setting determines how densely content is displayed and where field labels are located. Display density is controlled for the org in Setup, and users can also set display density to their liking from their profile menu.
What is variant in Lightning input field?
You can set a variant on lightning-input-field if you want specific fields to have a label and field alignment that’s different than that used by the form. A variant overrides the display density for that field.
What is display density in Lightning Experience?
In Lightning Experience, the display density setting determines how densely content is displayed and where field labels are located. Display density is controlled for the org in Setup, and users can also set display density to their liking from their profile menu.