If there is a limit, I don't know it, but as they increase the size of the template in which they are stored, there is the potential for that template to become unmanageable.

You could save your building block contents as documents and insert the documents as required. Then you are limited only by disc storage capacity - which effectively means unlimited. See Insert a selection of documents

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com