Sources don't agree on whether there was ever a 7th Imperial Guards Regiment.
Leland Ness indicates that there was, in fact, a 7th Imperial Guards Regiment that was raised during May 1943. He indicates that the 1st Imperial Guards Division (ex-Guards Mixed Brigade) formed in Japan in May 1943 was, in fact, an understrength square division, composed of four infantry regiments (1st, 2nd, 6th and 7th Guards Infantry Regiments), a cavalry regiment and an artillery regiment. This seems to make organisational sense - the organisation of the Guards Mixed Brigade was retained (a brigade of two infantry regiments, the 1st and 2nd), a second brigade of a further two infantry regiments (the 6th and 7th) was added, and a new artillery regiment was raised in addition to the extant cavalry regiment.
Ness indicates that the 1st Imperial Guards Division was initially formed at a very low strength, with the infantry regiments only authorised 1,716 men apiece in two battalions (i.e., a total of eight infantry battalions - barely equal to three normal-strength infantry regiments). The newly-formed artillery regiment only authorised 1,031 men in two artillery battalions of three batteries each.
The initial five Guards Infantry Regiments that Japan started the war with were all A1 regiments, i.e., draft, and each composed of three infantry battalions. The 1st and 2nd Guards Infantry Regiments, therefore, had a total of six infantry battalions. I think what happened in May 1943 is that the 1st and 2nd Guards Infantry Regiments each shed one battalion to form the core of the new 6th and 7th Guards Infantry Regiments.
It is entirely possible that, in time, a ninth infantry battalion was then raised for the 1st Imperial Guards Division, at which stage the four two-battalion regiments were re-organised as three three-battalion regiments, at which point the 7th Imperial Guards Regiment became defunct, converting the 1st Imperial Guards Division to a triangular division.
If the above did happen, presumably this occurred after 6 July 1944 (which is when the 3rd Imperial Guards Division was raised, with the 8th, 9th, and 10th Guards Infantry Regiments).
The above seems to me a logical way to explain the numbering issue around the Guards Infantry Regiments, and the disagreement on whether the 7th Guards Infantry Regiment existed.
So presumably Japan starts the war with fifteen Imperial Guard infantry battalions across five regiments. In May 1943, this is expanded to seventeen infantry battalions across seven regiments. By July 1944, this is increased to 26 infantry battalions across ten regiments; it is assumed that Japan ends the war with 27 Imperial Guard infantry battalions across nine regiments.