It depends on whether your bought them all at once, or purchased them on multiple days. Either way, you figure the basis per share and then multiply by the number of shares sold.
For example. if you bought 100 shares on 3/10/19, for $1000, your basis is $10 per share. The basis for 5 shares is $50.
If they are bought on different days, you account for oldest shares first (unless you have specifically identified the shares to be sold prior to selling them).
For example if you bought 2 shares on 3/10/19 for $23 and 5 shares on 3/12/19 for $42 and then sold 5 shares this year, here is how the basis is calculated:
$23 / 2 shares = $11.50 per share
$42/ 5 shares = $8.40 per share
2 shares at $11.50 = $23
3 shares at $8.40 = $25.20
$23 + $25.20 = $48.20 basis of first 5 shares
You list the 5 shares as purchased on Various as the date and $48.20 as the basis.