How many coronavirus particles are in the body at the peak?
This measure changes dramatically between viruses and infected individual but there are some estimates that have been done.

Bar-On, Flamholz, Phillips &amp; Milo, Apr 2020, eLife
From MERS and SARS (similiar to the current coronavirus) we know that from each infected cell emerge around 10^3 virions and it takes them 10 hours from the infection of the cell. We cannot deduce that every 10 hours the magnitude of virions in the body increases by 3 because:

Not all virions infect a new cell
Some virions infect the same cell and the burst size depends greatly on the cell
Not all virions are viable or able to infect

Another paper from Sender et al. (Nov, 2020) that is still in preprint (this is a long time for Coronavirus related articles) estimates the number of virions in peak infection at 10e9-10e11.
This estimate was done by analyzing data from Rhesus macaques (a monkey).
Concentration of virions per tissue weight was multiplied by the tissue weight to get total amount of virions.
On the relevance of Coronavirus information
Since our knowledge of Coronavirus advances and changes all the time it is important to note the date and source of information. Also, to advance science many papers published preprint articles regarding Coronavirus. This articles are published before peer-review and should be taken with a grain of salt as every peer-reviewd paper gets many revisions before published to the public.