Theory is often read in the 1NC as a shell. Here, the 1NC shell will be criticizing some practice of the 1AC. Often, 1NC shells are about the advocacy of the 1AC in the form of topicality or spec shells.
The 1NC can read as many shells as they can fit into their speech, which could realistically be as many as four shells. However, by reading more shells, you risk being able to spend less time on substance and developing the abuse story of each individual shell. The 1AR might choose to answer all of your shells together by contesting the paradigm issues rather than the abuse story of each shell specifically, which could be a waste of time for the 1NC.
1NC shells can be strategic because many debaters are often uncomfortable with theory debates, especially frivolous shells (obscure shells with minimal, unnecessary abuse stories), because it requires a lot of tech in a short 1AR to efficiently respond to all of the shells. Additionally, theory are often harder to generate offense against, especially when the 2NR can spend all of their time collapsing to one shell, whereas the 1AR can only afford to spend so much time answering one. Additionally, the only offense the 1AR can generate against 1NC shells is at best an RVI or some meta-theory argument, which makes it a no-risk strategy for the negative. In fact, it is often strategic for the negative to read theory just as a backup out in case they are unable to collapse to substance in the 2NR.
However, there can be some strategic downfalls to the practice of reading 1NC theory, since the 1AR can easily group all the shells by answering paradigm issues or cross apply generic standards like critical thinking and terminal defense, which makes it a poor time tradeoff from the perspective of the negative. Additionally, judges who are not receptive to frivolous theory and will hack against theory heavy 1NCs, or at the very least tank speaks.
Here is an example of an NC that contains many theory shells. Each shell is different, and there are still four separate ways to go for substance if the theory debate is lost.