How many bags of blown-in cellulose insulation do I need
There is no generic coverage chart for cellulose insulation -- or any other loose fill insulation for that matter. You will have to consult the coverage chart for the specific product you intend to install. On the chart you will find rows for R13, 19, 22, 30, 38 and 49. Some manufacturers provide additional R-value information. None of these rows will give you exactly R49 assuming an existing R25, but you can make a reasonable estimate. If your existing insulation is loose fill fiber glass there will be some compression if you put cellulose on top of it. In that case you may want to arbitrarily choose to install an additional R-30. The final insulation deck will cedrtainly be somewhere between R50 and R55.