US army has 10 active divisions, each with a Brigade of 3 artillery battalions each equipped with around 24 howitzers (either self-propelled M109A7 or towed M777) & 9 MLRS or HIMARS. Thats 720 howitzers and 270 MLRS, for a total of around 1000 pieces.
The National Guard has a further 8 Brigades, making around another 800 pieces for a grand total of about 1800 pieces including about 500 MLRS..
That is, of course, only those currently assigned, not those held in reserve. The army having, over the years, bought over 990 MLRS.
A lot are mothballed, but a lot are also being refurbished and updated. e.g.
https://www.lockheedmartin.com/en-us/products/m270.html
Not saying they can afford to give a lot away - but they have more than you suggest.