I'm assuming that you are using basic HTTP authentication which means you are using a password database cretedd/modified with htpasswd.
The htpasswd DBs are generic, which means it's not bound to a repo or anything. So you can easily do following things:
1. create a one DB for all SVN users and point to that DB for all repos
2. use Access control lists to specify which users have read and write access to the repos
This method is not too flexible, but will work for basic needs. If you need more flexible authentication needs you might have to look into other methods.
I assume that you know how to create password files and add/edit/remove users. So here's what you can do. Use htpasswd to create a file which can be used for all the repos. You can point to it using the AuthUserFile directive.
AuthName "Subversion repos"
The AuthzSVNAccessFile directive can be used to specify the Access Controls you want for the repos. In the above example the ACL file is /etc/svn-acl-conf which will have content similar to:
staff = joe, george
john = r
@staff = rw
The last example have a repository named "framework" and repo path is /repo. There's a group defined as "staff" which consists of users joe and george. The group staff have read-write accress to framework repo, which the user john only have read-only access.
Hope this helps you. For more info refer this
. While the two links discuss specific cases, you might be able to get something from them.