In (1), both of these sentences are more or less OK, depending on where you want to put the emphasis. However, it's possibly an odd way of phrasing what you want to say: maybe something like je ne suis pas allé à beaucoup de fêtes, je n'ai pas participé à beaucoup de fêtes would be more usual?
In (2), you need the feminine form vingt et une. In general, the feminine form is always used when un and vingt et un (and trente et un, quatre-vingt-un etc) come directly before the noun or noun phrase. Where you will find variation is when the number isn't directly before a noun or noun phrase. So there's variation between, say, quinze heures vingt et un and quinze heures vingt et une.
By the way, even though it makes no different to pronunciation, you'd usually write heures in the plural (and in general, write the noun in the plural: vingt et une personnes...).