Adobe Creative suite 5 re-installing on MacBook.

>i know this is an old software but Is there any download site available ?

As far as I know, Adobe has removed that very old version from their site... BUT, try this link

Go here for SOME older links https://prodesigntools.com/adobe-direct-download-links.html
Prodesigntools is an official Adobe partner, and the links go to Adobe, not a 3rd party site

>I don't have the activation serial number.

You can't do ANYTHING without the serial number

Log-in to your Adobe account and click this link https://account.adobe.com/products
-click Order history on the left and then the link to your order that will show on the right
-and finally click the link "Downloads and serial numbers" that will appear

-this will only work if you registered your serial number when you had it

as mentioned already, there's no way to install 32 bit apps on a 64 bit only os.  you would need to install parallels or something similar with a compatible os.

also, you would need to recover your serial number.  there are several possible ways to find/recover an adobe serial number.

1. search purchased materials: https://helpx.adobe.com/download-install/kb/find-serial-number.html

2. you can check your account (if you purchased from or registered with adobe AND you remember the adobe id used to purchase/register), https://accounts.adobe.com

3. if your adobe program is installed on another computer you can recover its serial number from that computer,

(win:) http://www.belarc.com/free_download.html

(mac:) https://mac-product-key-finder.com/

Unfortunately, older Adobe software like Creative suite 5 won't work on any Mac running macOS Catalina 10.15 or newer operating system because Apple deprecated the running of 32 bit apps which of CS5 has some including the installer itself.

https://support.apple.com/en-us/103076

Is your new MacBook new or if it's used what year is it?

If it's a new MacBook or recent used one, then you can subscribe to Photoshop using the Creative Cloud Photography Plan for about $10 a month, which includes Photoshop, Lightroom and several other related applications.

https://www.adobe.com/creativecloud/photography/compare-plans.html