What is a normal percentage of miniaturized hairs in a non balding man?

About 10% would be in miniaturization. Hair loss needs an in-person evaluation. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Scalp hair miniaturization is a sign of active hair loss in men and women with patterned baldness. Healthy people may have up to 20% miniaturization that is a sign of recycling of scalp hair and is not necessarily a sign of active hair loss.

It is natural for human to lose daily 5-100 hair shaft in telogen phase, and it is an expected outcome of hair cycle. These hairs appear again and grow back. About 10% of all hair on our heads is in the telogen phase and they would be in a miniaturized structure.

The normal scalp contains less than 10 % telogen vellus hairs based on extensive studies done by Rebora and colleagues.  Seeing 3 - 8 % is very normal in a healthy scalp.ReferenceDistinguishing androgenetic alopecia from chronic telogen effluvium when associated in the same patient: a simple noninvasive method.
Rebora A, et al. Arch Dermatol. 2005.

I have published a paper on this subject and reported that up to 20% of hairs may be miniaturized as part of the normal telogen-anagen cycle