o Is there a limit on the number of covariates which can be included in an multiple analysis of variance?
Not really. Whereas SPSS limits you to 10 in ANOVA, the limit is 200 in MANOVA -- more than appropriate for nearly all research situations. As one adds covariates, the likelihood of collinearity of the variables increases, adding little to the percent of variance explained (R-squared) and making interpretation of the standard errors of the individual covariates difficult.