Are lab reports suppose to be double spaced or single spaced

Formal lab reports are usually single spaced under the appropriate headers. These are usually:
Title
Abstract
Introduction
Materials and Methods
Results
Discussion
Conclusion
References

Unless otherwise stated by your professor.

Formal reports and submissions are usually done single spaced. You should go by your prof's preference. Most prof's like double spaced because it leaves room for comment.

Usually singe spaced, here is a link to some standard formats:
https://drive.google.com/templates?q=lab+report&sort=hottest&view=public