We are about create my sites for our users. In this regard I have some questions. How many my sites can we have in a web-application for my site? How many AD sources can one have with regards to Active Directory and My Site in SharePoint 2013 ?
in this case msdn is you best friend. As i created it for the company that i work for i could answer this for you ;)
mysite limmit for webapplication
first you need to understand what is a mysite? a mysite is a site collection.
The mysite when setup if done according to best practice should be hosted on its own web application. This web application would be the central storage of all personal mysites. Each individual user would have a mysite directly under the webapplication which is basicly a site collection for that user.
so now we know that mysite is a web application and that each users mysite is a site collection we can now find out the max size. A farm can hold 750,000 site collections out of 750,000 is 500,000 personal site collections. Lets not forget that is huge!! this would mean that your content database would be massive.
for a big business with over 500,000 employees would require a different setup with a sharepoit farm (need multiple shared farms) to accept 1milion users+! as this would be a massive strain on the farm and is highly not recommended. The chances of you getting to this value is remote unless you work for a massive global organisation like a bank!
Active directory sources
This depends on your topology of your network but in theory you should be able to add as much as you need! Once again its based to your topoloy and how AD is setup on how many forrests and domains you have and the trust between them.
its not hard todo but a pain to get right! in my case i did some STSADM code to get multiple forrests to connect together within sharepoint when searching for users in peoplepicker.
key here is that you have your user service configured properly so that multiple domains and/or forests are searchable for user profile import and from this they can create their mysite.
sync one way(import ad)
Maximum supported limit is 750,000 MySites (site collections) per web application. I don't think there is a limit. There is nothing on TechNet that says otherwise, but I'm sure the number you have is much less than SharePoint can handle. If users are planning on using their MySites for content, I would recommend creating multiple MySite content databases, depending on how much space a user would have in their quota for space. Just remember that 200GB is the supported size for read/write operations in a normal web application, unless you are in a medium sized organization with a fast SAN and virtual server environment, then it's 4TB.
For example (under a 200GB limit):
100 content databases for 10,000 mysites can allow 1,000 people spread across all content databases, and each user having a maximum quota of 200 MB.