The changes in the draft instructions for the 2023 5500-SF say:
IV. Changes to 2023 Instructions for Form 5500-SF Short Form Annual Return/Report of Employee Benefit Plan
1. Instructions for Form 5500-SF, “General Instructions,” “Who May File Form 5500-SF,” numbered paragraph 1 is revised to add two new sentences at the end to read as follows.
1. The plan (a) covered fewer than 100 participants at the beginning of the plan year 2023, or (b) under 29 CFR 2520.103-1(d) was eligible to and filed as a small plan for plan year 2022 and did not cover more than 120 participants at the beginning of plan year 2023 (see instructions for line 5 on counting the number of participants). To determine the number of participants covered by defined benefit pension plans and welfare plans, use the number described on Form 5500-SF, line 5a. Defined contribution pension plans use the number described on the Form 5500-SF, line 5c(1), except use the number described on line 5c(2) for defined contribution pension plans that check the “first return/report” box on Part I, line B;
Similar wording appears in the Form 5500 instructions.
Short version, if you check the box that says this is the first filing, the count to determine if the plan is a small plan or a large plan is based on the end of year count that is reported on line 5c(2).