Originally 22, as far as I can discover. Cut to 12 original spec ships, with the intention of ordering an improved Mogami in place of the cancelled originals. 10 have been laid down, 9 launched so far. All scheduled to be commissioned by 2028. The first 10 are being built FFBNW VLS, but funding has been allocated for 16 Mk41 VLS to be fitted to them. 133x16.3m, 3900 tons standard, 5250 full load,
Last year, Mitsubishi said it had been selected as prime contractor for 12 of the next frigate class, "New FFM", 142m x 17.4m, 6200 tons full load, 32 Mk41 VLS, etc.
Japan's MHI shows 'New FFM' Frigate at Indopacific 2023
This year that was renamed "upgraded Mogami" & slimmed down slightly to 17.0m beam. 4880 tons standard, perhaps about 6000 full load. Building is planned to start next year (2025), two launched in 2027, three delivered by 2029 & all commissioned by 2036.
"Upgraded Mogami" Is The New "New FFM", Looking At Australia
All that's reported as from either Japanese official sources or MHI, so it's as good as you're likely to get, & seems rather solid.
Perhaps all these ships will be equipped with the new A-sam, supposedly derived from the Type 03 Chu-sam, but it's slimmer & lighter, & with a booster. This (Korean?) link says the naval variant is called the Type 23
Type 03 surface-to-air missile