Thank you very much, as for your question, I meant that I have an univariate logistic regression model (i.e., with only one dependent binary variable), where the dependent variable must be explained by a number of binary independent variables (1,0). I have no problem when the independent variables are continuous in nature and normally distributed, because there is Hsieh (1998) who said that you can obtain the total sample size basing on the multiple correlation coefficient between Xi and the remaining predictors... However I didn't find anything like that for the model that I talked about above. So I hope to find in APPLIED LOGISTIC REGRESSION what I looking for.