Websites with numerous subdomains or totally separate domains

Of the two options above, I would use subdomains; e.g. service1.maincompany.com to attempt to keep the SEO benefits concentrated in one place. However, unless there is a compelling reason to use subdomains you probably shouldn't.

You should only use separate domains if you have different brands with different marketing efforts. If your marketing uses your company name, then you should have all your products and services on your company's domain name.

I would prefer to see it done even a third way.

I would have one domain name e.g. maincompany.com and set the services up as category pages within the main site keeping the integrity of the main site the same and it would also have better authority overall as one larger site. Folders vs subdomains as proposed in option 2 above.

Sub-pages could then be added too under those as well, ensuring your page titles showed in the URL to maximise SEO value. Therefor maincompany.com/services/plumbing/drain-laying.html (e.g.) would give far better results (assuming the page itself was rich in content and used the targeted keywords a few times within that page.