How many reviews do I need?

Enough to look established and recent enough to look active. A steady trickle beats a burst of many reviews in one week, which looks manufactured.

Can I ask by WhatsApp broadcast?

Ask individually to customers who bought, not by mass broadcast to your whole contact list.

They influence local visibility and click through rate strongly, and they give AI assistants something to cite when recommending businesses.

Yes, on the pages where people decide. Keep them genuine and attributable.

Usually the ask is too late, too long, or the link is broken on mobile. Test the link yourself before blaming the customers.