Table 1: Authorised Strength
Table 1 shows the authorised or establishment strength of the US Army. The regulars had a massive expansion (on 1st April 1898 it had only 28,183 officers and men) and never made authorised strength. The volunteers did make roughly authorised strength over a few months.
Table 2: Strength of US Army by last day of the month
Unfortunately the tables I extract this from don't divide the regular artillery into light artillery with guns (on wars outbreak 10 batteries ISTR, 2 each from 1st-5th US Arty), heavy artillery manning fortifications (6th and 7th US Arty) and those acting as infantry (majority of 1st-5th US Arty).
By the end of August 274,712 officers and men had enrolled, meaning the US basically met it's target.
The strength of the 7 army corps stood up (VIth Corps was never activated) was:
Table 3: Strength of Corps (Enlisted only)