If it's Zoom, you buy meeting space for 100. Then pay extra to up to 500 and more again for a max of 1,000. The add-on for 500 is £60-odd per month, so if you are on limited budgets, this can add up.
I think the question must be asked............Ooooooooooooooooooooooooo's gonna pay for it?
Indeed ... the average salary in this country is £30,000 per annum ... £140 per day ... and it takes somewhere between 5 and 10 times the length of a training session to prepare material. Yes, I know that's a ball park figure and can vary wildly.
It strikes me you have the investment of producing material, and also the admin setup of managing bookings, etc - much easier now with things like Eventbrite, but still not inconsiderable, and it seems a shame having made all that setup investment not to run a matinee as well as an evening show, or however you would define it, where the repeat cost would be a tiny fraction
(Option a). First run costs a lot then you just turn the handle tuning as you go - that's simplified, but it's a career / business lesson I followed with some success!
Sure, extra £60 per month is a big step up from £10 for 100. But you are looking at an organisation account with a salaried team, not volunteers. In perspective, average annual salary based on that £30k above is £700 per week, or monthly wage bill of around £50,000 (and that's before employment taxes and cost of providing working environment) across 17 staff.
Option b is a bigger session with this extra cost £60 cost
I would personally commend the idea of running each session (for they are mainly training sessions not interactive meetings) multiple times -
option a. The setup investment is there - "please turn the handle not ones but perhaps 2, 3 or even 4 times". Groups may be smaller, but that would give better opportunity for delegate interaction, as well as allowing everyone who was keen enough to want to attend to do so.