You can sign as many as you want, as long as you have room to back count to the previous year, and up to 28. Numbers always work themselves out, and with a new coach there is always attrition.
There are 2 limits -- 25/year and 85 total. If there are more than 60 on scholarship, then you can't sign 25 that year. It always depends on redshirts and attrition.