I went over to the USA three times this year, for roughly five days at a time. Now I am here for the summer, to travel and enjoy vacation before I go home in September, which will be 87 days. Are the other visits included in the 90 days? If so that would make it 102 days. My country is under the visa waiver program.
The US 90-day limit is generally per visit, but there is an exception that involves making short trips from the US to "contiguous territory" or "adjacent islands" (that is, to Canada, Mexico, or the Caribbean).
Regardless, you don't have to worry about that if you're already in the US, because when you were admitted to the US, the immigration officer will have written or stamped a date in your passport that indicates when you must leave the US.
If for some reason the officer neglected to do that, you can look it up online at https://i94.cbp.dhs.gov.
(Those who are not in the US and want to plan a trip, assuming they're not in the middle of a short trip to another North American country, can count on 90 days, but should be aware that repeated longer visits with shorter periods separating them will eventually lead to greater scrutiny and possible refusal of entry.)