The visa C is issued for a stay of up to 90 days for a period of 6 months, for the period indicated in your letter of invitation. That stay in Serbia can be with a single entry, which would involve one entry and one exit in those 6 months, i.e. a continuous stay of up to 90 days. Also, you can “interrupt” your stay, i.e. have multiple entries in the approved period of 6 months, that is, enter and leave the Republic of Serbia several times, as long as your total number of days spent in Serbia does not exceed 90 days.