The only limit I know of is the number of simultaneous connections allowed - the total number of downloads is definitely not limited.
If you have a large number of machines on very slow links, then it is possible to use up all the allowed simultaneous connections - but as soon as these machines have completed, then other machines will be able to update.
How many machines are using these repositories?
What type of network link separates the repositories and the clients?
What type of machines are hosting the repositories, and what OS?