can i add fake “sold” number in the product page? so it make it look more reliably
You can, but you shouldn’t. Lying to your customers is wrong, and I’m not going to help you do it.
If you want to show REAL sales on your site, you can try this app (https://apps.shopify.com/better-sales-pop) which shows a little popup with information on the last product (and time) when you made a sale.
You can also use an app like “Vitals” (https://apps.shopify.com/vitals) to show sales countdowns, limited stock counts, ‘spin the wheel’ popups, and more – to try and engage with visitors to help promote sales.