The technical ceiling is not the one you hit first. Two other things bind earlier: the subscriber's feed and the account's behaviour.
The feed
Ten sources at five posts a day is fifty posts. No reader takes that: the channel goes muted, then unfollowed. And the people who leave are exactly the ones who came for the topic — they wanted selection, not a stream.
Count posts per day rather than sources, and keep that number to what a reader can absorb — how often to publish.
The account
The working account has to be a member of every source. Mass joining is a reason for a restriction in its own right, especially on a fresh account — joining without getting banned.
Hence: add sources gradually, not as a list on day one. Twenty joins in a row from a new account is close to a guaranteed pause.
Overlapping sources
Channels in one field publish the same things. With ten sources you will get the same news item four times in an hour. That is more noticeable than posting rarely, and more annoying.
How to choose
Three sources you read yourself and know the quality of beat twenty picked by name. Selection is what an aggregator is for; without it the channel is a folder of subscriptions.
How Neurogram does this
Everything above is manual work that runs into the number of hours in a day. Neurogram does it on your own accounts and without you: it collects channels and audiences, writes comments and replies with a language model, and keeps limits and quiet hours so the accounts survive. See pricing or look at your case.