As promised, here are the establishments set for the 9th Regiment of Foot for the period you are interested in. I have only the totals and not the breakdown between battalions. However in December 1806 the 2/9th Bn. had 400 men after the 1st Battalion was completed to its establishment and was urged to increase its recruiting parties.
Dated 26 January 1807:
For 25 December 1806 to 24 December 1807 - 1,610
Dated 28 March 1808:
For 25 December 1807 to 24 December 1808 - 2,461
Dated 29 March 1809:
For 25 December 1808 to 24 December 1809 - 2,289
The 1st Battalion embarked at Cork 17 June 1808 with 956 r & f.
The 2nd Battalion embarked at Ramsgate 19 July 1808 with 647 r & f.
If Rod can compile those company establishment figures, we may be able to see where the increases occurred and if the regiments formed depot companies over and above the 10 companies we usually think make up a British Regiment/Battalion.
Hope this helps.
Happy New Year.