There are multiple potential IP warming scenarios (and I could write a book full of them), but I'll focus on a common one. Assuming that you're a daily sender, limit yourself to somewhere around 20,000 to 50,000 messages per day for the first week-- start as low as you can stand to. Double that daily email volume each week. Meaning if you send 20,000 email messages per day in week one, send 40,000 email messages per day in week two. Continue to double that "daily limit" number every seven days.
Of course, I'm being a bit US-centric here, and a bit B2C centric. My 20,000 starting point assumes a general mix of US B2C (consumer) ISP subscribers which tend to be a heavy mix of Microsoft, Gmail and Verizon (Yahoo) followed by very small numbers of others. For B2B senders, keep in mind that IP warming is still needed. Microsoft and Google host a LOT of B2B mailboxes and those concerns over building up your sending reputation still apply.
If you ask five different deliverability consultants how best to plan and execute an IP warming strategy, you will probably get five different answers. See, ISPs don't really publish exact numbers, and each ISP is a bit different, so different smart people may have reverse engineered completely different guidance.
The primary goal here is to give ISPs a taste of what you're going to be sending now and in the future, aod for them to like what they see. Meaning favorable responses from their subscribers. Subscribers who engage in high numbers and report spam in low numbers. Send unwanted mail, and it's going to be tough to ever move beyond IP warming to solid inbox delivery.
Email authentication and technical best practices matter here as well. Use properly configured domains, ensure you've properly published SPF records, and make sure all mail is signed with DKIM authentication. Proper implementation of a DMARC record couldn't hurt either.
If you're pretty technically savvy, you might read this very brief overview and think to yourself, that's great, I can do this. If not -- if it reads like it's written in another language -- then it might be time to partner with a smart consultant or agency to get some help mapping out exactly what you should be doing and how you should be doing it. Don't be afraid to ask for help when you need it!
If you want to learn more, here are IP Warming guides from SendGrid, Braze, Sparkpost, Green Arrow Email, Adobe Campaign Classic and Salesforce Marketing Cloud, as well as a help article from Return Path.