How many Catholics are excommunicated in an average year? Giving the strict nature of so many Catholic  teachings it would seem that their would be a lot of folks who find abiding by the rules just to difficult.
Pre marital extramarital sex contraception use etc...
Are there significant numbers who are excommunicated and who makes the decision to do so?
I do not think that any hard numbers are kept about excommunications, because excommunication is intended to help persons repent of their sins, not to pillory them in public.

However, the short answer is that there are very few excommunications imposed or declared each year.

Note that none of the sins mentioned by the O.P. merit excommunication. Excommunication is reserved only for extremely grave offenses that, in the judgment of the Church, cause grave and lasting harm to the faithful or to society, and hence require special treatment in Church law.

Some background regarding excommunication

Excommunication is a penalty imposed for extremely grave offenses that Canon Law specifically treats as delicts or crimes. When applying the penalty, the Church does not consider only the severity of the offense, but also whether, given the circumstances and times, certain goods need special protection under the law. (See Catechism of the Catholic Church [CCC] 1463.)

For example, because unborn children are extremely vulnerable nowadays to abortion, the Church punishes those who procure abortion with an automatic excommunication. (There is debate among canonists as to what “procuring” abortion consists in, but all agree that the doctor performing it and the medical staff strictly necessary for the procedure to occur fall under this penalty; see Code of Canon Law [CIC] 1398.)

There are also penalties of excommunication for various sacrilegious acts, such as desecrating the Eucharist, revealing the secret of Confession, assaulting the Pope, and so on. (There is actually a fairly complete list on Wikipedia, for the curious. It seems to omit the relatively new penalty for attempting to ordain women.)

The effects of excommunication

It is important to remember that excommunication does not remove a person’s membership in the Church. It merely imposes a penalty which, among other things, forbids that person from receiving the Sacraments. (It also, for example, prevents a person from holding an office in the Church.) Communion can be restored by repenting of the action accomplished, and seeking reconciliation from a priest (or bishop) with the necessary faculties to lift the excommunication. Also, any person may receive the Sacrament of Reconciliation when he is in danger of death. (In such a case, if he recovers, although his excommunication has been lifted, he is obliged to make recourse to the competent Church authority as soon as he can reasonably do so.)

See CIC 1331, for more information on the effects of this penalty.

Latae sententiae vs. ferendae sententiae

An exact answer to the O.P.’s question is complicated by the fact that the Canon Law contemplates two kinds of excommunication: one that is incurred simply by committing an act (latae sententiae); and the other, imposed by a competent Church authority (ferendae sententiae).

Latae sententiae excommunications are generally not known to the general public, and public records are not kept about them. Reconciliation with the Church is done entirely in the so-called “internal forum”—that is, secretly, in the confessional. Hence, it is extremely difficult to estimate the number.

(My personal estimate, as a priest, is that the number is relatively low, because a number of stringent conditions are necessary for the excommunication to occur: the person must be an adult, must be free from grave fear, and must be fully aware that excommunication results from his actions.)

Latae sententiae excommunications, in particularly grave and public circumstances, can be declared—as happened in the case of Archbishop Marcel Lefebvre and Bishop Antônio de Castro Mayer, and the four bishops consecrated by them, in 1988.

An excommunication can, on the other hand, be ferendae sententiae; that is, imposed as a penalty by competent Church authority.

Counting declared latae sententiae and all ferendae sententiae excommunications together, there is only a small number each year, especially considering the sheer size of the Church—some 1.3 billion according to CARA.

(There is, oddly enough, a list kept on Wikipedia of Catholics excommunicated by year.)