Hello there Rowaida Subhi We do not provide any specifications on a load balancer that you would look to utilize. I would assume that ones like an F5 load balancer would function for what you are looking to perform.
You can see based off our documentation that we refer to the load balancers as third party load balancers:
I am sorry we cannot provide recommendations.
I can't find any documentation that describes load balancing to PORTAL for ArcGIS. All the scenarios described in ESRI documentation seem a bit dated. The link you provided describes connecting a load balancer to ArcGIS Server. I want to connect ArcGIS Enterprise base deployment to load balancer. Do you have any description/information for that workflow?
The information for Portal will be in the Configure a highly available Portal doc:
Configure the LB:
The main things for a load balancer are setting the X-Forwarded-Host header and an HTTP health check against the healthCheck URL.
The deployment strategies documentation outlines different architecture: