That's more or less accurate, although the NEC's New Haven-Boston electrification went variable-distance between substations.
NEC 25 kV substation listing:
-- Branford @ MP 80, powering MP 73.6 New Haven (12.5 kV to 25 kV phase break) to MP 103.1 Westbrook* [29.5 miles]
-- New London @ MP 123.6, powering MP 103.1 Westbrook* to MP 150.2 Richmond* [47.1 miles]
-- Warwick @ MP 177, powering MP 150.2 Richmond* to MP 198.7 Norton* [48.5 miles]
-- Sharon @ MP 212.4, powering MP 198.7 Norton* to Boston South Station terminal district (roughly MP 228.5 + terminal trackage) [29.8 miles]
*locations of switching stations, which isolate each substation's power sections.
The substations are all built off the ROW requiring offsite local road access, and obviously need to be placed near enough to a set of transmission lines to tap them. The Shoreline's switching stations are built on or adjacent to the ROW and require no external access, but take up the equivalent of 2 track berths and thus may require strips of property acquisition for a tri- or quad-track section. Each power section bordered on one end by a substation and the other end by a switching station is then broken up by paralleling stations once every 6 miles located within the ROW containing the circuit breakers and single autotransformers. Those are laid out as wide as the switching stations (i.e. about 2 track berths), but are only about half as long...basically similar in size to any old biggish set of signal bungalows.
-- Mill River/New Haven phase break to Branford sub - 0 paralleling stations (total distance < 6 mi.)
-- Branford sub to Westbrook switching station - 3 par. stations
-- Westbrook switch to New London sub - 2 par. stations
-- New London sub to Richmond switch - 4 par. stations
-- Richmond switch to Warwick sub - 3 par. stations
-- Warwick sub to Norton switch - 3 par. stations
-- Norton switch to Sharon sub - 1 par. station
-- Sharon sub to South Station terminal district - 2 par. stations (Readville par. is the only paralleling station NHV-BOS that features 2 autotransformers instead of 1)
Note that Sharon and Warwick subs are only half-filled with equipment...at only enough equipment to run Amtrak equipment on 2 tracks. Slack space is fully future-proofed within the substation security fences for adding the MBTA/RIDOT's and SLE's shares of transmission equipment for commuter rail capacity, and at 4 full electrified tracks when NEC FUTURE expands out the capacity inside MBTA/RIDOT Providence Line territory. The switching and paralleling stations appear to be set up to be expanded back-to-back on that same approx. 2-track footprint for any future tri- or quad- track considerations, so would be expanded by length not width.
So...basically 30-mile long power sections for sections that contain a big terminal district, and 50-mile long power sections everywhere there isn't a terminal district (incl. regular end-of-line commuter rail layovers that lack shops). If space for the sub & switching equipment is provisioned with foresight like they did with the NEC, spacing on the Hudson Line would be roughly the same for 2 vs. 4 tracks and dense vs. less-dense commuter traffic. You can hedge a little bit on tighter spacing in current MNRR electric territory because Croton-Harmon is a full-blown terminal district with shops, but don't go overboard and assume that twice the traffic in MNRR electric territory requires twice the number of 25 kV subs. Distances between electrical section breaks are not so rigidly fixed that you have to go shopping for more sites instead of bigger sites like the NEC Shoreline has done to provision for more tracks and electric commuter rail density. Hudson Line will probably need more square feet per substation site than the max-provisioned Shoreline subs to stuff more equipment, but you'll never be so pinned in for +X square footage at any one candidate site that you'd be forced to build wholly additional substations too-short a distance away. Not when the sub spacing is flexible enough that you can just shop around for a different site in the same area that's got +1 extra acres of land to be had inside the security fence, and when the switching and paralleling stations will already fit on what's a contiguous 5+ track ROW Spuyten Duyvil to Hastings-on-Hudson and contiguous quad everywhere SD to Barrytown except for the rock cliffs pinch between Peekskill and Garrison.
Use the NEC layout of subs + switching stations as a guide to come up with your Hudson/Empire layout. Figure that because of the Croton-Harmon terminal district you'd need a shorter 25-30 mile distance to an adjacent sub on the immediate power section that contains Harmon shops. And also that you'd need a shorter 25-30 mile distance to an adjacent sub on the immediate power section that contains Albany shops. But everything in-between--including MNRR Upper Hudson on 3 electrified tracks in former diesel territory--can probably be spaced at 50 miles. Including Poughkeepsie layover, since that extra storage wiring is going to host far less activity than a terminal w/shops like CH or ALB or a heavy inner-vs.-outer zone transfer station like CH. Note also that the Hudson layout does not have to take into account the Penn Station terminal district, which is on the 25 Hz side of the Empire Connection phase break at MP 1.2...or GCT and anything inbound of Hudson MP 11, which would remain on the existing 750V DC third rail south of CP 12 interlocking where the Empire and Hudson merge.
Just mapping out a few spacing scenarios in my head...I keep arriving at a consistent 4 subs for spanning the whole corridor to ALB. Definitely no more than 5; that's shorting way too many power sections by way too much if you're arriving at a count of 6 subs or greater.